Tekkai is one of the six techniques of "Rokushiki".
The Tekkai hardens the users' muscles to the level of iron, in order to nullify damage taken from attacks. However, it can be broken by strong enough forces such as Luffy's various Gears, Zoro's Shishi Sonson and Tatsumaki, Sanji's Diable Jambe, Franky's Strong Hammer, and Chopper's Monster Point.
When Tekkai is active, the user is unable to move, with the exception of Jabra's Tekkai Kenpo. Tekkai can also be used to enhance the users' attacks by the increased density one gets from using Tekkai, similar to Busoshoku Haki, although aside from Jabra, it requires the user to utilize Tekkai after they are mobile towards the opponent.
The strength of Tekkai can vary for users with different physical conditions, as a person with a higher level of muscular strength would be able to take stronger forces and enhance attacks with his/her use of Tekkai. Furthermore, despite the increased physical density that one gains when activating this move, one should remember that the user's body is still flesh and blood. Thus, its resistance to force has its limits. Franky, for example, used his actual iron fist to break through Fukuro's Tekkai, and Kumadori's Tekkai couldn't protect him from the insane strength of Chopper's Monster Point. Furthermore, Tekkai cannot harden the body against non-physical attacks, making the user as vulnerable as ever to certain attacks. An example was during Sanji's fight with Jabra where he used the burning force of friction to incinerate through the latter's Tekkai. As such, even masters of Tekkai are far from indestructible; the limit of one's Tekkai to resist damage does however improve with mastery. Rob Lucci was able to resist even cannonballs at the age of 13 and was able to easily withstand the same iron fist of Franky's that hurt Fukuro.
- Tekkai "Utsugi" (鉄塊「空木」 Tekkai "Utsugi"?, literally meaning "Iron Mass: 'Deutzia'"): Used by Lucci. This form of Tekkai is used entirely as a counter instead, where the purpose is not to absorb damage from the attack, but to transfer the shock of the attack back to the attacker, which can lead to broken bones in a normal human body. In the FUNimation dub and Viz Manga, this is called Iron Body Empty Tree. The kanji for the deutzia literally means "Air/Sky Tree", hence the FUNimation dub's translation.
- Tekkai "Mushikaku" (鉄塊「無死角」 Tekkai "Mushikaku"?, literally meaning "Iron Mass: 'No Weak Spots'"): Used by Kaku in his half-giraffe form. Kaku bends his neck and legs to shift himself into a cube. When Tekkai is used in this form, he eliminates the weak points of his body, particularly his neck, and ensures that not a single spot is vulnerable to his following Kiri Shigure, though he looks ridiculous to his opponents. The pun is that "mashikaku", means "perfect square", and again is a play on Kaku's name. In the FUNimation dub and Viz Manga, this is called Iron Body Edgewise.
- Tekkai Kenpo (鉄塊拳法 Tekkai Kenpō?, literally meaning "Iron Mass Fist Law"): Used by Jabra. Using Tekkai Kenpo, Jabra is able to move while using Tekkai. Tekkai Kenpo is also an entire fighting style, with various techniques of its own. In the Viz Manga, this is called Iron Body Kung Fu, and in the FUNimation dub, it is called Iron Body Kenpo. Kenpou is a popular term for basic self-defense Chinese martial arts.
- Tekkai Kenpo "Okami Hajiki" (鉄塊拳法「狼弾」 Tekkai Kenpō "Ōkami Hajiki"?, literally meaning "Iron Mass Fist Law: 'Wolf Bullet'"): After hardening his hands with Tekkai, Jabra punches his opponent. In the Viz Manga, this is called Iron Body Kung Fu Wolf Bullet. In the Funimation dub, this is called Iron Body Kenpo Wolf Repel.
- Tekkai Kenpo "Roga no Kamae" (鉄塊拳法「狼牙の構え」 Tekkai Kenpō "Rōga no Kamae"?, literally meaning "Iron Mass Fist Law: 'Stance of Wolf Fang'"): Simply called Rōga for short. Rōga is one of Jabra's forms of Tekkai Kenpō. The way that Rōga is called is a pun in Japanese; oftentimes sentences end with darō, approximately meaning "probably". Sometimes, however, the particle ga would be added onto the end of a sentence to make it seem more direct. This is the way Jabra normally speaks, so he simply replaces where he would normally say darō ga (だろうが) with da rōga (だ狼牙). In the Viz Manga, this is called Iron Body Kung Fu Wolf Fang Stance. In the Funimation dub, this is called Iron Body Kenpo Wolf Fang Stance.
- Tekkai Kenpo "Roba no Kamae" (鉄塊拳法「狼芭の構え」 Tekkai Kenpō "Rōba no Kamae"?, literally meaning "Iron Mass Fist Law: 'Stance of Wolf Banana'"): A form of Tekkai Kenpō, in which Jabra seems to combine Tekkai with Soru in order to move at extreme speeds. In the Viz Manga, this is called Iron Body Kung Fu Wolf Plantain Stance. The kanji is a possible pun on the term rouba (老婆, lit. old woman).
- Rokaru Area Network (狼狩エリア・ネットワーク Rōkaru Eria Nettowāku?, literally meaning "Wolf Hunting Area Network"): While moving at high speed, Jabra slashes at his opponent with both hands. Because of his high speed, it appears as if he was attacking with multiple hands. "Rokaru" is the Japanese way of pronouncing the word "local", making this attack a pun on "local area network". In the Viz Manga, this is called Wolf Hunt Highspeed Scratch.
- Tekkai Kenpo "Don Poro" (鉄塊拳法「重歩狼」 Tekkai Kenpō "Don Pōrō"?, literally meaning "Iron Mass Fist Law: 'Heavy Step Wolf'"): A punch to the opponent's stomach with Tekkai in use. "Dongpo Rou" can also mean simmered square pork meat in Chinese. In the Viz Manga, this is called Iron Body Kung Fu Don Po Ro.
- Tekkai Kenpo "Matenro" (鉄塊拳法「摩天狼」 Tekkai Kenpō "Matenrō"?, literally meaning "Iron Mass Fist Law: 'Demon Sirius/Heaven Wolf'"): Jabra flips onto his hands, then kicks his opponent up into the air. In the anime, this attack was portrayed as a jumping kick instead. "Matenrou" is also Japanese for "skyscraper", while "tenrou" (lit. heavenly wolf) is a term for the Sirius wolf. In the Viz Manga, this is called Iron Body Kung Fu Devil Wolf.
- Tekkai "Rin" (鉄塊「輪」 Tekkai "Rin"?, literally meaning "Iron Mass: 'Wheel/Ring'"): Used by Blueno. Blueno uses Tekkai and kicks one leg into the ground and starts rotating his legs at a fast speed, causing his legs to simultaneously slam into the ground on a set path toward his opponent. It also seems to incorporate Rankyaku, as the legs leave blue traces in the air like a normal Rankyaku shockwave and a glancing attack cut a small amount of Luffy's hair. In the Viz Manga, this is called Iron Body Wheel.
- Tekkai "Sai" (鉄塊「砕」 Tekkai "Sai"?, literally meaning "Iron Mass: 'Smash'"): Used by Blueno and Kibin. After using Geppo to project himself towards his opponent, Blueno uses Tekkai to harden his body, smashing the enemy. In the Viz Manga, this is called Iron Body Smash.
- Tekkai "Go" (鉄塊「剛」 Tekkai "Gō"?, literally meaning "Iron Mass: 'Strength'"): Used by Blueno and Kumadori. Visually, the move differs from the ordinary Tekkai in that the user visually tenses his body even further while standing still; the ordinary Tekkai could be activated seamlessly. According to Blueno, it is his strongest form of Tekkai. In the Viz Manga, this is called Iron Body Strength when used by Blueno, and Solid Iron Body when used by Kumadori, and in the FUNimation dub, it is called Iron Body Strength in both instances.
- Tekkai "Sansetsu Lariat" (鉄塊「三節ラリアット」 Tekkai "Sansetsu Rariatto"?, literally meaning "Iron Mass: 'Three Section Lariat'"): Used by Shuzo. Shuzo throws his arm back, hardens his body with Tekkai, and finally swings his arm forward. This attack is strong enough to counter one of Sanji's kicks, but its full effects are not shown due to it being deflected. The name of the technique most likely came from the fact that Shuzo's attacking arm has three sections due to it having two elbows instead of one.
- Though acknowledged as a Rokushiki master, Kalifa is never seen using Tekkai.
- Tekkai is similar to Busoshoku Haki, as both reinforce the user's body in some way, the difference being that Haki is based on the user's willpower and Tekkai is based on the user's physical power. Also, both Tekkai and Busoshoku Haki are capable of both offense and defense. During his fight with Luffy, Shuzo used Tekkai to power up his punches and match a Busoshoku Haki-enhanced Gear Second attack from Luffy as well.
- Tekkai appears to be based on the Iron Shirt technique in Chinese martial arts, which utilizes the natural energy (ki) to strengthen the body's resilience.