Radar data processing software is a complex system, which has been implemented with mixed language programming using Visual Studio .Net and Visual C++6.0. COM-based software architecture for radar data processing system is proposed in this paper. It provides modular design framework, and integrates modules developed with different programming language and ready-made modules. Firstly, introduce COM technology in brief, then design radar data processing software system framework, finally show the programming examples to explain how to define and refer the COM interfaces. The results show that using COM-based modular mixed language programming is an effective method to realize the radar data processing software system.