He is a former Shooto Lightweight Champion, and has also competed in the UFC, PRIDE, DREAM, Strikeforce, ONE FC, and Rizin Fighting Federation.

In the same year, Kawajiri was considered by many mixed martial arts publications to be the #1 Lightweight fighter in the world.

At the age of 19, Kawajiri viewed the 1997 K-1 World Grand Prix Semifinal match between Ernesto Hoost and Francisco Filho, persuading him to begin training in combat sports.

[4] Kawajiri's professional career started in Japan's Shooto organization where he lost to Takumi Nakayama via rear-naked choke submission at 2:44 of the first round.

He would then continue on a new winning streak eventually re-matching Ribeiro to capture the Lightweight Championship at Shooto's 2004 Year-End show.

Gomi ended up knocking down Kawajiri after a flurry of punches and proceeded to submit him with a rear-naked choke 7:42 into the first round.

He then made another PRIDE appearance in which he fought Chris Brennan and scored an even quicker victory 29 seconds in the first round by TKO.

There he lost a controversial decision to at the time undefeated American fighter and rising star in the Lightweight division, Gilbert Melendez.

Kawajiri used his strong wrestling and ground and pound to great effect, and he was also able to frustrate Cavalcante by matching him in the striking game on the feet.

Kawajiri faced Champion Shinya Aoki at DREAM 15 for the Lightweight Championship in a highly anticipated match up as the two are both from PRIDE and were meant to have fought in the now defunct organization.

Kawajiri faced UFC veteran Drew Fickett at DREAM: Japan GP Final.

Kawajiri rebounded from his loss against Strikeforce Champion Gilbert Melendez by pounding out Fickett (TKO).

After finishing two opponents consecutively, Kawajiri faced Kazuyuki Miyata at a tie-in event called Fight For Japan: Genki Desu Ka Omisoka 2011 promoted and presented by M-1 Global, DREAM, and the Inoki Genome Federation that took place on December 31, 2011, at the Saitama Super Arena in Saitama, Japan.

His next K-1 fight was at K-1 World Max 2009 Final 8 against Masato who defeated Kawajiri by TKO at 1:43 in the second round.

Kawajiri returned to form against former Strikeforce Lightweight Champion Josh Thomson, repeatedly taking him down and unleashing ground and pound, winning a clear unanimous decision.

In 2012, after long stints with Japanese MMA organizations and more specifically DREAM, Pride Fighting Championships, he signed a contract to an up-and-coming Vale Tudo, incorporated with Unified Rules, organization named ONE Fighting Championship - much like Pride - with its intent to revive that aspect of mixed martial arts, based out of Kallang, Singapore.