Abstract: During the operation of a hydraulic valve block, oil leakage faults occurred frequently. The causes of oil leakage in the valve block were analyzed by means of macroscopic observation, fluorescent penetrant testing, finite element analysis, fracture analysis, metallographic examination, chemical composition analysis and Brinell hardness testing. The results show that the crack in the faulty valve block originated from the intergranular corrosion defect at the transition arc of the electric spark hole. During operation, the crack propagated and extended to the surface of the valve block under the action of pulse oil pressure,ultimately causing oil leakage. The intergranular corrosion defect resulted from chromic acid anodic polarization treatment in the heat-affected zone of the electric spark hole.