The task of clustering Web sessions is to group Web sessions based on similarity and consists of maximizing the in-tra-group similarity while minimizing the inter-group similarity. The first and foremost question needed to be considered in clus-tering Web sessions is how to measure the similarity between Web sessions. However,there are many shortcomings in traditional measurements. This paper introduces a new method for measuring similarities between Web pages that takes into account not only the URL but also the viewing time of the visited Web page. Then we give a new method to measure the similarity of Web sessions using sequence alignment and the similarity of Web page access in detail. Experiments have proved that our method is valid and efficient. The task of clustering Web sessions is to group Web sessions based on similarity and consists of maximizing the in-tra-group similarity while minimizing the inter-group similarity. The first and foremost question needed to be considered in clus-tering Web sessions is how However, there are many shortcomings in traditional measurements. This paper introduces a new method for measuring similarities between web pages that takes into account not only the URL but also the viewing time of the visited Web page. Then we give a new method to measure the similarity of Web sessions using sequence alignment and the similarity of Web page access in detail. Experiments have cert that our method is valid and efficient.