This paper proposes a formal approach to modeling use case which captures requirements from multi-angle views: The class diagrams, the use case sequence diagrams, the use case state diagrams, the specification mapping and the system invariant. By defining formal semantics of those views, each aspect of requirements is given exact formal descriptions. As a result, integrated specification of one method can be built by integrating formal descriptions of its interaction specification and its functional specification. At the same time, properties of use case models can be specified and analyzed through the proof in design calculus. As an application, rules for checking the consistence of use case models are studied. An example to illustrate the feasibility of the proposed method is given.