While attending Shinil Junior High School in Seoul, Na was a power pitcher who could throw 88-mile fastballs. In, he was selected for the South Korean national youth baseball team. The team won the gold medal at the inaugural Asian Youth Baseball Championship, and Na was named MVP of the competition, appearing in all the games as a pitcher. In, he entered the Shinil High School baseball team, but converted to a position player due to elbow injuries. After three years at Shinil High School, Na went undrafted in the KBO Draft. Instead, he continued his baseball career in Dankook University. While attending college, Na was considered one of the top sluggers in the South Korean collegiate baseball league, winning home run titles in and. In October 2007, as a senior in Dankook University, Na was selected for the South Korea national baseball team to compete in the 2007 Baseball World Cup. He was the only amateur outfielder in the South Korean roster. In the tournament, Na batted.385 with 3 RBI as a starting left fielder.
Selected by the Kia Tigers with the 5th pick in the 2nd round of the 2008 KBO Draft, Na made his pro debut in the Tigers opening game against the Samsung Lions on March 29,. He was the first rookie player to bat cleanup in the opening game in the Tigers' history. In his first at-bat, Na struck out swinging against Bae Young-Soo. He was 0-for-3 with 1 RBI in his first professional game. Na fell into a slump, showing a weakness against breaking balls, and was demoted to the second team of the Tigers at the end of April. In June, he returned to the 26-man first team roster but played as a platoon or backup outfielder. Na finished his rookie season with a batting average of.295, 6 home runs and 30 RBI, playing in 73 games. In, Na became a fixture at left field for the Tigers as starting outfielder Lee Yong-Kyu was sidelined due to an ankle injury in the beginning of the season. His batting average dipped to.263, but he had 482 plate appearances, which enabled him to qualify for the batting title, and smacked 23 home runs. Besides, Na was 18th in walks, 19th in RBI and 23rd in slugging percentage. On October 24,, Na hit a game and series-winning solo home run in the bottom of the ninth inning off Chae Byung-Ryong during Game 7 of the 2009 Korean Series. The dramatic homer gave the Kia Tigers a 6–5 victory over the two-time defending champion SK Wyverns. Na's home run was the third walk-off home run in Korean Series history. Earlier in the game, when Kia was trailing 5–1 in the sixth inning, Na smacked a two-run home run off Wyverns' relieverLee Seung-Ho. Na's big bat in Game 7 helped earn him MVP honors in the Korean Series, although Tigers' starting pitcherAquilino López arguably deserved the award. Lopez was the winning pitcher in Game 1, a 5–3 Kia victory, then hurled a shutout in a 3–0 Tigers' win in Game 5. The former major-leaguer also got two outs in relief, with an SK runner on second base and the game tied at five, in the eighth inning of Game 7. In 2016, he qualified as a free agent and remained with a total of 4 billion won, including 1.6 billion won in four-year down payment and 600 million won in annual salary.
