In e-service environment, service contract is important for assurance of business interoperability and quality of services. Combining service contract and process model will facilitate analyzing service process and monitoring service execution. This paper proposes a service modeling approach consists of service contract and process model. Service contracts are used as service advertisement and service request in this approach. The operational semantics of service contract and process model are translated into Constraint Handling Rules(CHR) program by a compiler. Based on this approach, a concept named service contracting is defined as a phase between service discovery and execution, which checks the consistency of service advertisement and process model as well as the the applicability of services w.r.t. the consumer's capabilities. Further, constraint solver centered implementation is designed, the modeling and contracting of web services under this architecture is illustrated.