As a junior at Yongma High School in Masan, South Korea, Jang was selected by the Hyundai Unicorns with the 89th pick in the 2nd round (97th overall) of the 2002 KBO Draft.

In 2005, Jang was selected for the South Korea national team again, and participated in the Baseball World Cup held in the Netherlands.

In the 2006 KBO season, he finished runner-up to Ryu Hyun-Jin for Rookie of the Year with 2nd in strikeouts (142), 5th in ERA (2.85) and 7th in wins (12), and was selected to the All-Star team.

After the season, he was selected for the South Korea national team and competed in the 2006 Asian Games held in Doha, Qatar.

In the tournament, he pitched a 10−0 complete game shutout victory over the Netherlands and tossed 4.1 innings of scoreless relief against China.