Test complexity and test adequacy are frequently raised by software developers and testing agents. However, there is little research works at this aspect on specification-based testing at the use case description level. Thus, this research proposes an automatic test cases generator approach to reduce the test complexity and to enhance the percentage of test coverage. First, to support the infrastructure for performing automatic, this proposed approach refines the use cases using use case describing template and save it in the text file. Then, the saved file is input to the Algorithm of Control Flow Diagram (ACFD) to convert use case details to a control flow diagram. After that, the Proposed Tool of Generating Test Paths (PTGTP) is used to generate test cases from the control flow diagram. Finally, the genetic algorithm associated with transition coverage is adapted to optimize and evaluate the adequacy of such test cases. A money withdrawal use case in the ATM system is used as an ongoing case study. Preliminary results show that the generated test cases achieve high coverage with an optimal test case. This automatic test case generation approach is effective and efficient. Therefore, it could promote to use other test case coverage criteria.