Yoon So Yi debuted as a model in Graffiti Magazine in 2001. After a few years of print and commercial modelling, she auditioned for Ryu Seung Wan's action-comedy Arahan in 2004. Based on a popular South Korean comic, Arahan explored the consequences of unleashing an ancient evil spirit into a modern city, with Yoon's character part of a hidden school of ancient masters who train a naive young policeman to fight this evil force. Yoon underwent strict training for six months in a film action school, where she had to overcome her phobia of heights. The film was a relative commercial success, with one review describing her as "tall and attractive in an excellently non-cutesy way". She received a Best New Actress nomination at the 2004 Blue Dragon Film Awards. Yoon next starred in the TV drama Say You Love Me, a dark, twisted love story between four people. This led to a friendship with her co-star Kim Rae Won, with whom she starred in a series of advertisements for Samsung Digital Plaza. When Kim later launched his own management company Bless Entertainment, she would also join his stable of stars. After playing Jung Joon Ho's love interest in The Twins, Yoon was cast in Shadowless Sword, a wuxia martial arts epic set in ancient China. In the role of a beautiful young swordswoman who protects the future king, Yoon says that she was inspired by Anita Mui in The Heroic Trio and that the strong-willed female character appealed to her. She learned wushu for three months in Korea, then wire action from the Chinese stunt team. In 2006, she became part of the ensemble cast of Goodbye Solo, written by renowned drama writer Noh Hee Kyung. Even though her dramas do not attract a high number of viewers, Noh has gained a cult following for her in-depth portrayal of ordinary characters, taking a more human natural approach through realistic and yet emotional lines. Goodbye Solo featured seven characters of different generations and backgrounds, Watch drama online for free.