Prepared for his kickboxing career by K-1 fighter Nicholas Pettas, he debuted in K-1 fighting at the World GP 2005 event in Seoul in March, 2005, which he won in the final vs. Kaoklai Kaennorsing.

[10] He started the season 2006 at the K-1 World GP in Las Vegas with a unanimous decision win over Sylvester Terkay aka "The Predator".

[13] On August 5. at the Asia World Grand Prix in Hong Kong, he beat Gary Goodridge by KO at 1:34 in the first round.

On September 29 at the K-1 final eliminations in Seoul, fighting southpaw stance again, he redeemed his previous loss and defeated Mighty Mo by decision.

The win was disputed because Mo was able to land many hard hands on Choi's head, while the Korean missed most of his shots.

However, due to a kick Choi delivered to Mighty Mo's groin in the second round that was inexplicably ruled a knockdown.

In the post-fight Choi said: "He (Jérôme) has been my toughest opponent, who was able to resist my powerful punches and counter with speed, I admire his physical strength and I'm sure with more experience will be able to defeat him."

Notable: In the years 2008 and 2009, Hari fought 14 bouts and only lost 3 of them, one by disqualification against Remy Bonjasky, two by KO against Alistair Overeem and Semmy Schilt.

From his 11 wins, he won 8 times by KO in the first round, against Ray Sefo, Glaube Feitosa, Domagoj Ostojic, Frédéric Sinistra, Semmy Schilt, Zabit Samedov, Ruslan Karaev and Alistair Overeem.

Choi was picked as a first reserve fighter against Ray Sefo at the 2008 K-1 Championship finals which he lost by unanimous decision.

Sefo knocked out many opponents with the same right hand, including Jérôme Le Banner, Gary Goodridge and Ruslan Karaev.

[17] Choi was scheduled to face former WWE, IWGP, and NCAA national champion amateur wrestler Brock Lesnar at the combined promotion event K-1 Dynamite!!

[18] However, reportedly due to a benign tumor on his pituitary gland,[19] Choi was denied his California fighter's license on May 23, 2007, putting Dynamite!!

The sequence repeated itself, only the second time Fedor showed the difference in experience and locked the armbar, forcing Choi to tap at 1:54.

The fight was longer, as Cro Cop, a fellow K-1 specialist, found difficult to strike against the larger Choi.

He did land multiple low kicks through the round, which eventually caused enough damage for the fight to be stopped in a KO for Cro Cop.

He faced retired baseball player Jose Canseco in an MMA match on May 26, 2009, as part of its Super Hulk Tournament.

[22] The Korean advanced to the semi-finals at Dream 11 and was pitted against Ikuhisa Minowa, a Japanese fighter familiar with large differences in size.

Minowa resorted to creative takedown and guard pulls to bring the match to the mat, but Choi used his size and strength to shut down his gameplan.

[24] But many factors, including the issue that many top Korean stars such as Denis Kang and him (Choi) were not available in April, contributed to the event being canceled.

Even though he was technically very limited and mostly lost to the best K-1 and MMA fighters, his greatest strengths were his very good chin, overall punch and kick resistance, and a very strong Muay Thai knee strike.

Fighters like Carlos Toyota or David Mihajlov don't have a similar Knockout-ratio or punching power as Badr Hari, Jérôme Le Banner or Mighty Mo.

During his best days, Choi was also able to take all punches and kicks from Bob Sapp, Semmy Schilt and Remy Bonjasky without being knocked down or out once.

Schilt knocked out many of his opponents with punches and kicks, and Le Banner had with over 80% the highest knockout ratio in the entire K-1 Circus.

In July 2015, after 5 years and 9 months of inactivity in combat sport, he gave his comeback against Brazilian fighter Carlos Toyota, weighing 140 kg (310 lb), the lowest to this day.

[33] On November 15, 2018, he fought at the Chinese event MAS FIGHT against Wushu (sport) Fighter Yi Long, who weights 80 kg (180 lb) standing 1.76 m (5 ft 9 in).

[34] On June 11, 2019, he lost to David Mihajlov of Hungary, who sent him to the ground with several head shots 49 seconds into the first round, and the referee stopped the fight after a brief count.

On September 8 and 15, 2013, he appeared in episodes 310 and 311 of the variety show 2 Days & 1 Night (KBS2) as a guest of regular cast-member Kim Jong-min.

On November 16, 2014, he appeared in episode 28 of reality show Roommate (SBS) as a guest along with female singer Hong Jin-young.