With the rapid development of cloud computing technologies, there are increasing number of Web services deployed by service providers on cloud platforms. Invoking Web services is in the form of given orders, by which the end-user can process business and information in anytime and anyway. However, existing Web service compositions mainly focus on single Web services selection and neglect to consider the fact that complex services created and aggregated by service compositions might be low global QoS (Quality of service). In this paper, the importance and necessity to construct service clusters in business process are discussed. Then, how to build up a model to solve the optimal QoS problem of Web service composition using service clusters is proposed. Third, the dynamic programming algorithm is introduced to this model. Finally, a series of experiments are carried out to evaluate our method. The results show that service cluster has advantage to obtain composite service with best QoS for the end-user.