How to achieve the completeness and consistency of a software specification by construction is an important issue for software quality assurance, but it is still an open problem. The difficulty lies in the fact that the assurance of the completeness needs user's judgments and the specification keeps changing as requirements analysis progresses. To allow the user to easily make such judgments and to reduce chances for specification modification, in this paper we describe an intuitive, formal, and expressive specification method that integrates top-down decompositional and scenario-based compositional methods. The decompositional method is used at an informal level to achieve a complete coverage of the user's functional requirements, while the compositional method is used to precisely define the functionality of each scenario and to construct complex scenarios by composition of simple scenarios in a formal, intuitive language called SOFL. Combination of the decompositional and compositional processes will result in a complete specification in a hierarchical structure. We present an example to illustrate how the integrated method is used in practice and describe a software support tool for the method.