Abstract: In order to enhance the ability of diagnosing faults and improve maintenance,we develop a condition monitoring and main faults diagnosing system for a turbojetengine. The whole system is divided into two sub-systems, the engine operationmonitoring and the ground computer data processing. The former is used for analysing engine performance tendency and diagnosing some main faults in the engine, i.e., engine-surge, exceeding drop rpm, auto-flameout and ;so on. Inaddition, it can record the parameters at take-off or before and after fault occurs. When faults take place in the engine, it can also give pilots warning. The latter is used for analysing reasons of faults and estimating engine performance tendency.