Before competing in MMA, Yoon was a well-known judoka with over 50 international judo competitions under his belt and had a winning streak of 47 straight victories without conceding a single koka, with notable wins over World champions and Olympic gold-medalists such as Makoto Takimoto, Mark Huizinga,[1] Jeon Ki-Young and Toshihiko Koga.

Yoon made his MMA debut at PRIDE Total Elimination 2005 against legendary Japanese fighter Kazushi Sakuraba.

USA event, where he scored a surprising submission win over heavily favored Dutch kickboxer Melvin Manhoef.

His newfound propensity for finishing opponents with an armbar has led to fans and journalists dubbing the move the "Dongbar" when performed by Yoon.

Yoon then faced last minute replacement Tarec Saffiedine and won via split decision.