The process-driven composition of Web services is emerging as a promising approach to the integration of business applications within and across organizational boundaries. To support rapid and dynamic composition of services, Web services that meet users? functional requirements must be able to be located and bounded dynamically from a large and constantly changing number of service providers based on their Quality of Service (Qos). In order to enable quality-driven Web service selection, it is necessary to make a rational and effective decision among a number of similar Web services based on Qos criteria and users? preference. In this paper, it presents a decision model of Qos criteria called DQos for evaluating Web services, which consists of an extensible Qos model, decision modes and constraints. On basis of systematic studying Qos criteria of Web service, service selection is formulated as Multiple Attribute Decision Making (MADM) problem which can be solved by using subjective weight mode, single weight mode, objective weight mode and subjectiveobjective weight mode. At last, DQos model is proved to be feasible and effective by simulation experiments.