In this paper, 40Cr steel was gas-nitrided for varying time. The microstructure and microhardness of the nitrided layer were examined by means of optical microscope and microhardness tester. The CO2 corrosion behavior of 40Cr steel influenced by nitriding was studied using high temperature high pressure autoclave. The SEM, EDS and XRD technique were adopted to study the characteristics of CO2 corrosion scales on 40Cr steel. The results show that the pitting is generated for all the un-nitrided and nitrided specimens, however, the corrosion tendency and corrosion rate decreases with nitriding time increasing, mainly due to the compact of FeCO3 formed on the surface.