Actuated by motives such as reflecting the uncertainties existing in service invocations, conquering the sybjectivity of available measuring approachs quality of services, considering the internal relations among given factors, credibility, reliability, and availability of service invocation from different perspection is defined. By statistics historical information, the quantitative values of these factors can be obtained. According to the relevant fuzzy algebraic method, operator of compution more contributing factors is defined, given a funtion for measuring quality of Web service on unweighted factor. This approach is independent of the important of given factors and thus quite scalable.