This study investigates the effect of board diversity on firm performance in Korean manufacturing firms. Board diversity is defined in terms of the composition of directors’ age, university, and academic major and six diversity indices are adopted for this analysis. First, it is found that increase in board size does not lead to increase in board diversity. This finding shows that the positive relationship between board size and board diversity that previous papers have asserted does not apply to Korean firms. Second, diversity in directors’ academic majors is found to affect firm performance negatively; however, diversity in terms of age and university are not found to affect firm performance.