In the object-oriented software development process, use case diagrams of unified modeling language (UML) are applied to capture the user requirements. The traditional method of describing use cases is generally based on the developer's own experience to obtain use cases from the requirements manually. However, how to automatically generate accurate use cases is still a problem to be solved. This study proposes a method to generate use cases semi-automatically by using UML activity diagrams. Firstly, the study specifies the use case diagram and activity diagram by introducing a formal model, the unified structure. Secondly, it gives an algorithm for decomposing the activity diagram and then generates the event flow of the corresponding use cases, which is based on the dependency chain obtained from the decomposed activity diagram, to obtain the use case model. Finally, the case is demonstrated by the developed prototype CASE tool and the feasibility of the proposed method is verified.