Portal sites have been providing diverse services to attract new users and retain existing customers. Recently, they also added new services and increased quality of the services they provide to secure more customers. It is, however, not clear how people actually react to such activities. We wanted to understand the mechanism associated with users' continued usage intention of current Portal site. We assumed search, e-mail, information, community, e-commerce services as key services provided by most Portal sites and considered valuable by many customers. The quality of these services may directly affect users' continued usage intention. However, we can also assume that attractive alternatives and switching cost may have some influence on users' continued usage intention. Therefore, in this study, key variables pertaining to the quality of key services provided by Portal sites, attractive alternatives, and switching cost were examined to explain the dynamics of users' continued usage intention. An empirical study based on 891 survey responses showed the importance of the quality of key services for users' continued usage intention. Switching cost also demonstrated strong association with users' continued usage intention. However, attractive alternatives did not show a significant association with users' continued usage intention. We also found that the variable attractive alternatives moderated the relationship between the quality of search service and users' continued usage intention and the relationship between the quality of information service and users' continued usage intention. On the other hand, the variable switching cost showed a moderating effect on the relationship between the quality of e-mail service and users' continued usage intention and the relationship between the quality of community service and users' continued usage intention. This study implies that Portal sites need to make efforts not only to improve the quality of key services but also to consider attractiveness of other Portal sites' services and switching cost to secure more customers.