In component-based software engineering, software systems are mainly constructed based on reusable components, such as third-party components and in-house built components. Hence, system quality depends on the quality of the involved components. Any change of a component, it must be re-tested at the unit level, and re-integrated to form component-based application systems. Although a number of recently published papers address regression testing and maintenance of component-based systems, very few papers discuss how to identify component changes and impacts at the unit level, and find out the reusable test cases in a component?s test suite to support its evolution. This paper focuses on component APIbased changes and impacts, and proposes a systematic re-test method for software components based on a component APIbased test model. The proposed method has been implemented in a component test tool, known as COMPTest. It can be used to automatically identify component-based API changes and impacts, as well as reusable test cases in a component test suite. The paper also reports this tool and its application results.