Borrowing the thoughts of component interaction, the concept of Logic Component (LC) is proposed, and a Web application is divided into LCs which is mapped into the actual physical components finally. This paper supposes that each component and LC is a black box which has been well tested. So Web applications can be regarded as a set of interactive components. It mainly concentrates on testing the interactions of components. An automaton is used to model each component, and compositions of automata are used to model the component interaction. For each component-testsequence, a new automaton can be got by using composition of automata. Abstract test cases can be generated from the new automaton. By means of mapping the actions to the actual operations and adding the data of test space to them, the component test cases are generated.