Abstract: A review of the research in optimization order for fault diagnosis was discussed. The feasibility and advantage of using artificial intelligence AND/OR tree heuristic search were pointed out. The heuristic function consists of condition possibility of fault and search cost. The AND gate and OR gate fault condition possibility updown transfer rules, algorithem of heuristic search, branch cutting method were put forward. The quantization expression of diagnosis accessibility was given. The fault diagnosis of diesel engine lubricating system was illustrated.