Web service technology seamlessly enables integration of different software to fulfil dynamic business demands in a platform-neutral fashion. By means of standard interfaces, service requesters can obtain their desired functionalities through certain service discovery mechanism, and service providers can offer the functionality to service requesters without exposing details of provided services. However, the adoption of loosely coupled and distributed services will cause trustworthiness problems. In this paper, we use ontology to describe experience and we utilise statistical techniques along with Petri nets to conduct a trustworthy evaluation method for composite web services. The degree of trustworthiness of all involved services in an aggregated composite service will be evaluated by analysing past experiences so that the trustworthiness of the composite service could be estimated.