Abstract: An commercial vehicle engine suddenly stalled and could not start over a pre-sale inspection after long storage. After investigation, it was found that a timing chain fractured. The reason for fracture of the timing chain was studied by fracture morphology and microstructure observation, chemical composition analysis, hardness testing, and so on. The results show that during the storage, the stress corrosion fracture occured at one side of internal chain piece pin hole of the timing chain by the combination of vapor corrosion and tension stress, and then the fatigue fracture occured at the other side of the pin hole by working loads.