An expert system for analysing the performance and diagnosing faults of d.I. diesel engine is presented in this paper. This system consists of three parts. First is the transient data acquiring part by which the incylinder pressure, fuel pipe pressure and other parameters can be acquired through the A / D transforming circuits. Second is the simulative calculation and analysis part by which most of the parameters, such as the fuel injection rate, fuel heat release rate etc. can be calculated. Third is the expert diagnostic part established on the fuzzy theory, which is applied to evaluate the matching relations among various transient parameters. Some faults of engine can be detected by this part. Furthermore, the proposals to improve the performance of engine can be put forward by this system.