Quality of Web service is defined by a collection of attributes. A quality attribute expresses a capability or a requirement: – About the overall Web service, About a given aspect (operation, message) of the Web service. With the widespread proliferation of Web services, quality of service (QoS) will become a significant factor in distinguishing the success of service providers. The major requirements for supporting QoS in Web services are as follows: Availability: Availability is the quality aspect of whether the Web service is present or ready for immediate use. Availability represents the probability that a service is available. Accessibility: Accessibility is the quality aspect of a service that represents the degree it is capable of serving a Web service request. Performance: Performance is the quality aspect of Web service, which is measured in terms of throughput and latency. Higher throughput and lower latency values represent good performance of a Web service. Reliability: Reliability is the quality aspect of a Web service that represents the degree of being capable of maintaining the service and service quality. QoS determines the service usability and utility, both of which influence the popularity of the service. Quality of Service (QoS) is a major concern in the design and Management of a composite service. In this paper, a Systematic approach is proposed to calculate QoS for Composite services with complex structures. In particular, QoS Solutions are provided for unstructured conditional and loop Patterns. We also show how QoS-based service selection can be conducted based on the proposed QoS calculation.