This study intends to explore whether board structures characteristics have effect on the magnitude of extreme loss and return of firms' stocks traded on Taiwan Stock Exchange. We use value-at-risk (VaR) thresholds to quantify the potential extreme loss that a stock may incur, and then examine their relationship significance with four board structure variables, including board size, the percentage of outside directors, the duality of CEO and chairman of the board, and the expertise of independent directors. The empirical results show that board size, board independence, and independence expertise have significantly negative influence on the size of extreme loss, but less significantly similar effect on the size of extreme return. In addition, CEO power has positive effect on the size of extreme loss, but loses its significance of impact on the size of extreme return almost completely. The decline of impact significance on positive return side may imply that short-sell investors could feel freer than buy-long investors to monitoring board structure divergence for controlling their extreme losses.