Aiming at the deficiency of research on reliability evaluation of Web Services composition models, basing on the theories and methods of stochastic Petri net, this paper proposes an approach to reliability evaluation of web services composition based on business process execution language (BPEL). Firstly, the Web Services composition model is analyzed and built basing on the stochastic Petri net, and then the BPEL processes are transformed into composite service process Net model (CSPNet) correspondingly. Combining with the reliabilities of component services and the structures of CSPNet, the reliability formula of several composite services are obtained. Finally, a recursive algorithm is built to evaluate the reliability of Web Services composition. The experimental results show that the approach to reliability evaluation of web Services composition based on BPEL is simple, tractable and efficient.