Abstract: As an important basis of Web services selection, discovery and composition, the quality of Web services is widely studied and has become a subject of intense debate in recent years. It is necessary to conquer the subjectivity of existing methods for measuring quality of Web services, depict the inherent uncertainties implied in service invocations and the mutual relationships among the deterministic factors. Therefore, invocation rate, success rate and efficiency are defined as three representative factors with respect to the quality of elementary Web services. By statistical computation of historical invocations on elementary services, the quantitative values of the above three factors can be obtained. In this paper, according to the basic ideas of evidence combination and evidence superposition, an approach is proposed to measure the quality of elementary Web services based on the superposition of the above uncertain factors. Based on the mean of corresponding qualities of elementary Web services, another approach is proposed for measuring highergranularity services. Further, a method for determining priorities among Web services is given accordingly. The proposed methods are well consistent with people’s intuition. Performance studies verify their efficiency, scalability and feasibility.