While rapid progress has been made in the research and practice of hardware faul t diagnosis, the concept of software fault diagnosis, on the other hand, is not clearly defined yet, and diagnosing technologies of software fault have not been studied systematically and thoroughly. This paper compares the differences betw een software and hardware, analyzes the causes of software faults and the mechan ism of software failure, introduces various kinds of diagnosis technologies for hardware fault, and makes a preliminary investigation of software fault diagnosi s issue. 