With the rapid development of services and software, how to share, integrate and discover them properly in open and dynamic network environment is one of the most important challenges for software technology. With monad techniques, we present a novel formal semantic model for service oriented computing in a black-box observation way.The monad-based model can help us formally describe and further study on software components and services through monads' properties such as abstraction, reflection and composability. This model relatively improves service reuse and discovery, and it significantly facilitates web service composition and enables integration of legacy applications.