Nowadays, many research institutions home and abroad have conducted quite a few researches on QoS and Web services composition, and have made some valueable exploratory achievements. But as a new field of study, new research issues keep emerging. For example, how to make the estimation of QoS based on user preferences, how to validate and test in order to make the Web service correct, and how to choose the quality-driven services compostion sheme and sevices compostion based on the QoS requirements.In this paper, the solutions for services composition based on QoS are discussed in theory and practice. The main works by the author include the following four aspects:The reference matrix of QoS in SOC is proposed.The current QoS estimation methods are reserached and a kind of fuzzy method for QoS estimation based on the user requirement is put forward.The demand of Web services composition and its accuracy verified is gived. Web services are composited based on Pi-calculus, and the validity of Web services composite is verified using formal tools MWB.Analyzing the state of the art of Web services composition based on QoS, a new type of relationship matrix coding is proposed, and the optimization of composite services is made by using pseudo-parallel genetic algorithms, at the same time the relationship between businesss entities on QoS is considered when composing the services.In summary, we present well-evaluated solutions in this dissertation for some key issues of Web services composition. We believe that our contributions make a nice groundwork for future research and engineering on Web services composition both in theory and practice.