There’s seldom work carried on dealing with the contradiction between the vague and uncertain requirements of end-user and the precise and deterministic process in service composition. A multi-grain formal model for service composition is proposed in this paper. This model considers the requirement of customers in service composition from the end-user view and a formal specification on mapping the Web service description to the generalized decision logic language (GDL) is presented to construct multi-grain service composition views. GDL is a formal logic language proposed in granular computing research community as an expecting specification for definition of granular models. It can be used to define a multi-grain model for service composition and make users and service composition agent work in different information granule level separately. The proposed model is expected to provide a more understandable view for an end-user than traditional service composition model and conforms to the human cognition mode.