Abstract: Macroscopic inspection, fracture analysis, chemical composition analysis, mechanical property testing and metallographic examination were conducted to clarify the reason for fracture in hexagon head bolt. The results showed that hydrogen was not removed after acid pickling and electroplate, the vestige of hydrogen in the bolt and the stress caused the hexagon head bolt to fracture. Furthermore, there was the carburized layer about 0.2 mm,it improved the hardness and increased the sensitivity of hydrogen embrittlement of bolt.