Aiming to the character that page attribute of session pattern in Web server log is Boolean quantity, an algorithm of Web usage mining based on sequence number is presented. The algorithm turns session pattern of users into binary, and then uses the way of number ascending to search candidate frequent itemsets. The algorithm computes support by sequence number dimension in order to scan once session pattern of users, and then the efficiency of Web usage mining is efficient improved. The experiment indicates that the efficiency is faster and more efficient than presented algorithms.