Abstract: By means of macro and micro morphology observations, metallographic analysis, chemical composition analysis, EDS and hardness test for the factures of piston and cylinder liner, as well as water content analysis in oil in crankcase, the failure causes of the piston and cylinder liner for emergency diesel generator were investigated. The results show that without modification treatment, the eutectic aluminium-silicon alloy of the piston which had coarse flake primary silicon and long needle-like eutectic silicon made it easy to break away from the matrix and then lower its plasticity and wearability. Under the combined action of cylinder score due to poor lubrication, the disintegration of the piston was accelerated and the piston fragments impacted onto the cylinder liner, finally caused the local fracture of the case.