He was born in Chonburi province, eastern Thailand, along his brother Tappaya Sit-Or and his nepehew Rambaa Somdet he trained in Muay Thai since childhood.
He was backed by his promoter Songchai, to turn to professional boxing career in 1994 along with Pichit Chor Siriwat in the junior flyweight division with Cuba's Ismael Salas as a trainer.
He would defend his title at four times, defeating famous boxers, such as Aquiles Guzmán and Jesús Kiki Rojas.
He later traveled to Japan where he lost the title to Satoshi Iida a Japanese boxer, an old rival who he drew with in the past, by unanimous decision at the Aichi Prefectural Gymnasium in Nagoya on December 23, 1997.
Yokthai competed in his lone mixed martial arts match against Shinya Aoki in 2010, where he lost via first-round submission.