Web site usability is a critical metric for assessing the quality of the B2C Web site. A measure of usability must not only provide a rating for a specific Web site, but also should it illuminate the specific strengths and weaknesses about site design. In this paper, the usability and usability evaluation of B2C Web site are described. A comprehensive set of usability guidelines developed by Microsoft (MUG) is revised and utilized. The index and sub index comprising these guidelines are present firstly. The weights of each indexes and sub indexes are decided by AHP(Analytical Hierarchy Process). Base on the investigation data, a mathematic arithmetic is proposed to calculate the grade of each B2C Web site. The illustrated example shows that the evaluation approach of this paper is very effective.