The rotational temperature of a nitrogen molecule (N2) was observed to increase proportionally with the substrate‐holder temperature in a capacitively coupled rf glow discharge system. The rotational temperature, equivalent to the gas temperature, from the second positive system of the emission spectrum of N2increased with increasing the mixing ratio of hydrogen, which correlates with the deposition rate, optical band gap, and etching rate of silicon nitride (SiNx). This technique can be used for monitoring the temperature of a SiNxsurface during glow discharge processing without any damage to the plasma.