The amended Act on Financial Investment Services and Capital Markets Act stipulates that listed firms with assets of 2 trillion won or more should have at least one woman on their boards. This study aims to explore the impact of board gender diversity on firm value and the role of institutional blockholders on the relationship. The results are as follows. First, the board gender diversity is positively related to firm value regardless of markets, firm size, industry. Second, foreign blockholders reinforce the positive relationship between board gender diversity and firm value, while domestic blockholders mitigate the relationship. Third, board gender diversity and firm value has an inverted U shape relationship. Firm value increases with the proportion of female directors to a certain level, after which it decreases. The optimal board structure was reached when the ratio of female directors was between 18 and 44 percent, up to conditions. This finding provides an insightful grounds on the effectiveness of the revised Act, regarding appointment of female directors and gives a guideline for policy makers by suggesting optimal level of board gender structure.