It is not optimal for firms with high information asymmetry to invite monitoring from independent directors because it is costly for the firms to transfer firm-specific information to outsiders. On the basis of this idea, we develop and test the hypothesis that firms with high information asymmetry have less positive impact of board independence on firm value. Using a comprehensive sample of 3,836 firms from 1999 to 2006 in Korea, where a regulation requiring outside directors was instituted after the Asian financial crisis, we find that the effects of independent outside directors on firm value are more strongly positive when the information asymmetry is low. For the analysis, we choose various information asymmetry variables including measures derived from two econometric models that are widely used in the market microstructure literature: the Huang and Stoll model (1997; HS) and the Hasbrouck (1991), Foster and Viswanathan (1993) model (HFV). Also, the impact of independent outside directors is more evident when firms face volatile situations such as higher growth opportunities, risks, R&D expenses, low credit rating, or under financial distress. Taken together, the information asymmetry can be a significant factor in determining the valuation impacts of independent outside directors.