Today people increasingly expect more from the functionality of a web site, so the web usability has emerged as an important topic. It is generally hard to renew the existing web sites to meet the changing demands of users. Therefore, the present study is aimed at detailing the usability problems of web sites. To this end, Heuristic Evaluation (HE) is used to identify the usability problems, and Analytic Hierarchy Process (AHP) is used to rate their severity. Finally, for a more user-friendly web site, a new approach to judging severity of usability problems is developed by integrating AHP into HE.