The future digital television will provide viewers new interactive services, e.g., Electronic Programming Guide, TV shopping, video-on-demand, etc. The set-top box needs a middleware to launch these applications before interaction and collect garbage after quitting or switching to another service. It is the application manager's responsibility to manage the lifecycles of the services. Thus, the application manager plays a crucial role in controlling the applications and efficiently utilizing the limited resources of set-top box. This paper presents a design of an application manager, including a lifecycle model of an application and a communication protocol between an application and the application manager. Also, the Application Information Table (AIT) which carries the essential application signaling information in a transport stream is introduced. Finally, the implementation, which uses Java technology, is described in detail.