The quality of service (QoS) is a major concern in the design and management of Web service composition. Existing methods for QoS calculation either do not take the probability of path execution into consideration when QoSs are provided for different execution paths, or do not take different execution paths into consideration when a single integrated QoS is provided for the whole composition. In this paper, a comprehensive QoS analysis approach is proposed that calculates the QoS probability distribution by considering both the execution probability and execution conditions of each path in the service composition. Four types of basic composition patterns in the service composition are discussed: sequential, parallel, loop and conditional. In particular, a QoS solution is provided for all types of loop structured service composition.