The purpose of this study is to investigate the effect of carbon content on the formation of nitride layer onto the plasma nitrided Cr-Mo Steels. Plasma ion-nitriding was performed for SCM415, SCM420 and SCM440 at temperature ranges from 525℃ to 625℃, for 1,4,9 and 16hours, in the atmosphere of 80% N₂ and 20% H₂ gases in vol.%. In case of plasma ion-nitrocarburising, CH₄ gas of 1 and 3 vol.% was mixed with N₂ gas. Optical microscope, scanning electron microscope, X-ray diffractometer, Auger electron spectroscope and micro-vickers hardness tester were used to examine the characteristics of nitrided layer. It was found that grain boundary diffusion process played an important role in the rapid growth of compound layer at the early stage of ion-nitriding reaction. In plasma ion-nitriding, the compound layer of SCM440 was thicker than those of SCM415 and SCM420. However, the thickness of compound layer was decreased for all specimens in plasma ion-nitrocarburising. The amount of micropores in the compound layer of the nitrocarburised specimen was found to be quite diminished and the case depth was decreased with increasing carbon content in both the matrix and the carrier gas.