Abstract: The leakage reason for some QT400-18 nodular cast iron pipe used in gas pipeline was analyzed by means of macro and micro analysis. According to the macro analysis results it can be concluded that the leakage of the pipeline was caused by local corrosion and the corrosive leakage developed from inwall to outward until piercing. Micro cracks and other defect were not found by microstructure analysis. Large S element was contained in the corrosion products of pipeline inwall according to component analysis results, which indicated that the corrosive leakage of the pipeline related to the S element in the carried gas. The comprehensive analysis results show that the leakage was as a result of local electrochemical corrosion caused by the impurity of S and condensate water existed in the pipe. Suggestion was proposed to control the impurity and water in technics range.