This paper presents a novel approach of generating system testing automatic test cases from UML design diagrams. This method can be applied to any system modeled by UML. Associating Graph Theory innovatively, we abstract use case and sequence diagram and define coverage criteria to generate test cases in our test case generation scheme. Through two levels traverse, generate test cases automatically and satisfy the coverage criteria stated in this paper. The test cases thus generated are suitable for system testing and to detect operational, use case dependency, interaction and scenario faults. The coverage reaches at its maximum level, covering all use cases, use case dependency, and each sequence of message path of interaction.