Internet-Based Web application systems are gradually built as software service coordination systems. In an open, dynamic and changeful application environment, trust is an important thing for security and reliability of software services and systems. In this paper, first an Agent-based software service coordination model is presented. Then a trust valuation model is given to value trust relationships between software services. Trust is abstracted as a function of subjective expectation and objective experience, and a reasonable method is provided to combine the direct experience and the indirect experience from others. In comparison with an other’s work, a complete trust valuation model is designed, and its reasonability and operability is emphasized. This model can be used in coordination and security decision between software services.