Opening domestic market to international trade may enhance not only actual competition but also potential competition from foreign competitors. It seems that the competition authority has focused mainly on the actual competition (measured by the current market share) and has paid less attention to the potential competition. In this regards, this paper investigated the relation between potential foreign competition and domestic market structure. Using dynamic panel regression model, we analyzed the dynamic response of import penetration to the changes of domestic market condition in Korea as a proxy for the degree of potential foreign competition. The empirical results suggests that potential foreign competition does exist in the Korean manufacturing sector and this tendency is more stronger when the market is more concentrated. Thus, in order to effectively implement competition policy, it is necessary to consider both actual and potential competition.