The mechanism of the formation of active nitrogen atoms and the function of hydrogen molecules for the dissociation of the mixture of nitrogen and hydrogen in the plasma nitriding process are discussed in this paper. The results show that the nitrided case thickness of nitrided samples become thinner and the surface hardness decreases with the increase of cathode voltage. Active nitrogen atoms mainly result from non-elastic collision between positive ions H2+ and neutral particles N2 for N2-H2 mixture plasma nitriding at 550V cathode voltage, the volume percent of e-Fe3N and the superficial nitrogen concentration of plasma nitrided case decrease with the increase of the mixture ratio of N2/H2. However, active nitrogen atoms mainly result from non-elastic collision between energetic positive ions H2+, N2+and neutral particles N2 for N2-H2 mixture plasma nitriding at 650V cathode voltage. The existence of a few of nitrogen molecules makes critical energy of positive ions H2+ dissociating neutral N2 molecules drop to 26.06eV and the cathode voltage threshold value become 490V.