While attending Yonsei University, Na was considered the best hitting pitcher in the South Korean collegiate baseball league.

As a freshman in 2008 Na batted a team-leading .329 with 11 RBI and posted a 3–1 record and a 1.70 ERA with 83 strikeouts (runner-up in the collegiate league) in 74.0 innings pitched.

As a sophomore in 2009 Na led the collegiate league in home runs (4) as a cleanup hitter and amassed 7 wins (2 complete games – 1 shutout) with a 2.93 ERA as a starting pitcher.

In September 2011, Na was called up to the South Korean national team again for the 2011 Baseball World Cup held in Panama.

In 2012, he played 94 games in the farm league, finishing with a .303 batting average, 16 home runs, and 67 RBI.