Properly generated test suites may not only locate the defects in software systems, but also help in reducing the high cost associated with software testing. It is often desired that test sequences in a test suite can be automatically generated to achieve required test coverage. However, automatic test sequence generation remains a major problem in software testing. This paper proposes an Ant Colony Optimization approach to automatic test sequence generation for state-based software testing. The proposed approach can directly use UML artifacts to automatically generate test sequences to achieve required test coverage.