A number of success story about various application areas including manufacturing, accounting, finance, education, and engineering are reported. MIS professionals predicted that expert systems would improve the productivity enormously. However, the expert system revolution has not happened yet. Although not reported in the open society widely, there are failure stories of expert systems. Most of problems concerning expert system failure stem from the non-technical issues such as cognitive and psychological problems rather than the technical issues. We hypothesize that human factor principle enables designers to handle most of these non-technical problems elegantly and to improve the performance and acceptance of the expert systems. Major reasons for expert system failure and needs of human factors are discussed. Human factor guidelines to expert system make the prospects of the expert systems with human factors clear and understandable.