C38 carbon steel has been nitrided by radiofrequency cold plasma discharge with the aim to study the effect of gas composition on structural and mechanical properties of nitrided substrates. The plasma nitriding was carried out on unheated substrates, using two different gas mixtures (75% N2/25% H2 and 25% N2/75% H2) for 8 h of plasma treatment. Electron Probe Microanalysis (EPMA) showed that the nitrogen content and the nitride layer thickness mainly depend on the hydrogen content in the plasma. For 75% N2/25% H2 mixtures, nitrided samples showed the formation of thicker nitride layer with high content of nitrogen compared to those formed in the case of 25% N2/75% H2 or pure N2 . The thickness of the nitride layer formed is approximately 15 μm for 8 h of plasma with 75% N2/25% H2 . X-ray photoelectron spectroscopy (XPS) was employed to obtain chemical-state information of the plasma- nitrided steel surfaces. The mechanical properties of plasma-nitrided C38 steel were investigated by Vickers microhardness measurements. The micromechanical results show the formation of a new hard layer on the surface after N2/H2 treatment plasma and it reaches a maximum value of more than 2242 HV0.005 for 75% N2/25% H2 plasma-nitrided samples. However, a decrease in the hardness values with the applied load has been evidenced.