Employing a panel data set that combines Statistics Korea’s ‘Mining andManufacturing Survey 2004-2008’ with ‘Current State of the National TradeUnion Organizations 2006-2008’, we examine impacts of unions on theestablishment’s employment size, wages and productivity. We use fixed-effectsmethods to overcome limitations of previous research that relies oncross-sectional data. We find that unions in small establishments with lessthan 30 employees have little impact on the establishment’s employment size,wages and productivity. Effects of unions on establishments with at least 30employees differ by the measure of the performance. In particular, unionsraise the average wage of the establishment with at least 100 employees by2.1% to 12.1%.