Video on demand (VOD) system already applies widely in many domains with the rapid development of multimedia and network technique. In the DirectShow based VOD system, the role of DirectShow is to decode and play streaming media. In VOD, the kernel technology includes server- side document management technology, and client server network multimedia transmission technology, client data playback of multimedia technology, multimedia technology and multimedia data cache decoding technology. In DirectShow, the most kernel technique is that COM modules work as a filter and insert into standard components. It is the COM object that executes specific tasks. This paper research the kernel techniques of DirectShow based VOD system according to COM modules, such as multimedia document management, broadcast, buffer and data decoding.