In this paper, we propose FECT, a new modelling framework for describing and composing heterogenous Web services to satisfy emergent requirements. In FECT, a three-dimension description pattern, i.e. the Environment, the Capability and the behavior Traces, has been proposed to describe Web services. FECT has the following features, which may distinguish it from other work. Firstly, the environment, with which Web services may interact, has been introduced for characterizing the context of Web services. Secondly, the capability of a Web service is captured as the effects on its environment. Thirdly, the composition process is accomplished by combining the traces of Web services.