Abstract: The engine block leaked during testing in South China Sea. Appearance observation of gas channel and fracture surface, energy spectrum analysis, microstructure inspection, and hardness testing were performed. The results show that the cracks of block are stress corrosion cracks. The highly-concentrated chloride ion on surface of gas channel, and the high sensitivity to stress corrosion caused the stress corrosion cracking of the engine block. The stress corrosion failure of the block can be prevented by carefully cleaning to reduce the remained chloride ion as well as improving the material state or changing material to lower the sensitivity to stress corrosion.