Abstract: A fuel-hydraulic oil radiator was found to leak in use. The failure mode and cause were analyzed by macro and micro observation, microstructure examination and welding depth testing. The results show that the circumferential welding crack between output head and shell body is fatigue crack, which was caused by incomplete fusion welding defects. The axial welding crack of shell body is fatigue crack, which was caused by welding porosity and unqualified melting depth. The leak of the fuel-hydraulic oil radiator was caused by the welding fatigue crack between output head and shell body. The shell body welding fatigue crack brought in larger pressure and tympany, which promoted the leakage of the radiator.