A community chatroom with instant message and file transfer is developed. P2P communication technology is introduced and composite P2P architecture is used in the development. P2P mode and C/S mode are combined, server node play the role of promoting coordination among nodes and function expansion. Data structure and database are designed. Interface, messaging, file transferring, debugging are completed and experiences are put forward. Many functions of C# are used smartly in the programming. Network resources are efficiently utilized, the burdens on server and network are reduced because communications among users are performed directly in the system.