In surface nitriding by plasma, a major concern is to enhance the density of reactive species of nitrogen. One method is to mix a gas with an ionization potential lower than that of nitrogen. The hydrogen gas mixing was carried out in an electron cyclotron resonance (ECR) nitrogen plasma. Relative density of molecular ion in the vicinity of a substrate was measured by an optical emission spectroscopy. Under a fixed discharge pressure, the densities of nitrogen molecular ion (N-2(+)) and excited molecular nitrogen (N-2*) were observed: they have a maximum, when a ratio of hydrogen pressure to nitrogen pressure was 0.5. Silicon nitriding was also conducted by using the nitrogen-hydrogen mixed plasma. In the case of maximum densities of reactive species, silicon nitriding was most effective.