Abstract: The possibility of using BPN to quantitative diagnosis of turbofan engine gas path component faults is investigated.By organizing the fault samples quantitatively,the construction of BPN is built and applied to the fault diagnosis of a low bypass ratio turbofan engine.The diagnostic results show that BPN can not only isolate the gas path component faults,but also identify the severity of component performance deterioration.It is proved that the BPN is adaptive to random errors in measure system and different engine operation regimes for a control plan.But the construction of BPN must be rebuilt under the condition of different engine control plans.Because it has a little ability of self-organization,it is diffcult for BPN to isolate the engine faults quantitatively when several faults occur at the same time.