In this study, the variation of hydrogen contents in gear steel 17Cr2Ni2MoVNb after gas carburizing, low-pressure carburizing and heat treatment was investigated by means of thermal desorption spectrometry (TDS) analysis. The results showed that there was an additional peak at around 255 ℃ in the TDS curves for the specimens after gas carburizing and quenching in comparison with that for the specimen after low-pressure carburizing, which was absorbed presumably from the high temperature environmental gas. The results of fatigue tests showed that the fatigue limits for the specimens after gas carburizing with the additional hydrogen desorption peak and after low-pressure carburizing without the additional hydrogen desorption peak were both about 980 MPa. It could be attributed to the fact that the absorbed hydrogen content in the gas-carburized specimens was decreased from 0.084 mass ppm to 0.057 mass ppm after fatigue tests, which was too small to have evident effect on the fatigue properties of the steel.