Abstract: The effect of gas nitriding treatment on the sensitivity of hydrogen embrittlement of carbon steel had been investigated by observing the microstructure and fracture models by using of scanning electron microcopy (SEM) and optical microscopy (OM). The results showed that the fracture mechanism of nitriding layer was hydrogen embrittlement but ductile fracture existed in substance area of the sample. It was concluded that the gas nitriding increased the sensitivity of hydrogen embrittlement in carbon steel.