As a way to compose separate services together to fulfill a function, service workflow is widely accepted and applied in different applications. One key issue in service workflow is how to configure the general service workflow into a specialized one that can satisfy user?s individual requirements. A service composition model based on constraints is introduced in the paper. In this model user?s requirement is modeled as constraints, which is further categorized into constraint related to static quality index and dynamic quality index type. The configuration engine can search the service according to the constraint model so that an abstract service workflow can be transformed into the concrete service workflow by binding each activity with concrete service. The framework for service composition together with how to represent the user?s requirement as constraints and the configuration process are introduced.