Service component architecture (SCA) is a new promising programming model for constructing service-oriented application, which facilitates the development of business integration in SOA. However, there still lacks the unified theory foundations for the service component assembly based on SCA in a rigorous way. Firstly, a metamodel of the contract-based SCA is proposed by applying the idea of design by contract to the assembly process of service components. Secondly, a signature model for the contract-based SCA is given according to the metamodel, which supports the construction of service-oriented application in a rigorous way. Moreover, reasoning about or verifying the correctness of service components assembly can be implemented based on the semantics of the port protocol. Finally, the online shop case study is discussed to illustrate how to construct a service-oriented application in a rigorous way.