The study suggests an answer to the question of what determines category diversification of Internet portals in Korea. First, as external factors, competition intensity and market growth are hypothesized to have influence on the degree of category diversification. Second, an internal factor, user loyalty to portals, is hypothesized to influence negatively category diversification. The study performed empirical analysis based on weekly portal-specific panel data of eighteen internet portals in Korea during the period between 2001 and 2004. The result shows that category diversification increases as competition intensity increases, and that category diversification decreases as user loyalty increases. There was no effect of market-level growth rate on category diversification.