After making his debut for the South Korea U-20 in August 2002, Choi was called up to the South Korea U-23 squad for the Asian Games, where he helped the side finished third place, earning a bronze medal. He then returned to South Korea U-20 squad, being called up for the AFC Youth Championship in Qatar. He started in the tournament and helped Japan reached the final. Choi started in the final against Japan U-20 and beat them 1–0 to help South Korea U-20 win the AFC Youth Championship. A year later, he was called up to the South Korea U-20 squad for the FIFA World Youth Championship despite suffering an injury that could have seen him being dropped from the squad. Choi made two appearances in the tournament, including one against Japan in a 2–1 loss in the Round of 16. Despite making two appearances, FIFA said about his performance on the website: "Korea’s outstanding number ten was the focal point for a formidable Taeguk Warriors’ attack. His brilliantly flicked goal in the Round of Sixteen match with archrivals Japan looked to have sealed a spot in the Last Eight. But a late flurry of attack from the Nipponese ended the dream. Nevertheless, Choi Sung Kuk’s constantly clever creative play was a delight, and the Ulsan Hyundai player will be one to watch in the years to come."