In September 2006, Yang was selected for the South Korea junior national team alongside lefty rival Kim Kwang-Hyun that won the gold medal at the 2006 World Junior Baseball Championship in Cuba. In the round-robin, he pitched four-scoreless innings against the Netherlands as a starting pitcher, racking up eight strikeouts. Yang started for South Korea in the semifinal game against Canada, facing future Mariners' pitching prospect Phillippe Aumont, and hurled three innings of two-hit, one-run ball with two strikeouts to help lead South Korea to a 6–1 victory. Yang posted a 1.04 ERA in 17.1 innings pitched in the tournament, and his 18 strikeouts were the second-highest total on the team after fellow left-hander Kim Kwang-Hyun (22 strikeouts). Yang was named to the All-Star team as the best left-handed pitcher.
Upon graduation from Dongsung High School, Yang was selected by the Kia Tigers in the 2nd round (1st pick, 9th overall) of the 2007 KBO Draft. He made his KBO debut on April 7, 2007 against the LG Twins at Jamsil Baseball Stadium in Seoul, coming from the bullpen, but allowed a solo home run in one inning pitched. He made his first start on April 12 against the Hyundai Unicorns and tossed 5.2 scoreless innings of four-hit ball. However, Yang struggled with his command and was never able to regain it to the level he had earlier in the season. At the end of May, Yang was eventually demoted to the reserver team of the Tigers. In August, the young left-hander came back to the Tigers first-team roster and pitched in relief of the Tigers' starters in the second half of the season. On September 29, 2007, Yang recorded his first professional win against the Hanwha Eagles in Daejeon, pitching five innings of two-hit, one-run ball as a starter. Yang finished his rookie season with a record of 1–2 and an ERA of 4.17 in 49.2 innings pitched, appearing in 31 games as a utility pitcher.
Yang had a disappointing sophomore season in 2008, slipping to 0–5 with a 5.83 ERA. He started the season as a starting pitcher, but struggled through five appearances with an ERA of 7.00 and was eventually relegated to the bullpen. Though frequently sent to the mound as a starter to fill a temporary void in the rotation during the season, Yang mainly pitched in relief for the Tigers, garnering 5 holds as a setup man.
In 2009, Yang had a career year. He was 9th in wins (12), 5th in ERA (3.15), 4th in strikeouts (139) and 12th in innings pitched (148.2), all of which were career bests. On August 11, he had one of his best performances of his pro career, hurling 7.1 shutout innings in which he only allowed 5 hits with no walks and struck out 10 batters against the Lotte Giants. On November 14, Yang pitched as the starter for the Tigers in the 2009 KBO-NPB Club Championship game against the Yomiuri Giants. In that game, he allowed only three hits and one run and racked up 6 strikeouts in 5.2 innings.
In 2010, Yang's ERA rose over a full point to 4.25 and gave up the second highest number of hits in the KBO league with 169, resulting in a .272 batting average against. However, he had four more wins than the previous year, posting a 16–8 record and striking out 145 batters. Yang was runner-up in wins and 3rd in strikeouts among the KBO pitchers in the 2010 season. He pitched his first career complete game shutout against the Samsung Lions on June 2.