Hydrogenated amorphous silicon nitride (a‐SiNx:H) films have been prepared by plasma‐enchanced chemical‐vapor deposition with a mixture of SiH4and NH3at a substrate temperature of 250 and 300 °C. The properties of these films have been investigated using x‐ray diffraction, infrared absorption, photoluminescence, and electron probe microanalysis. From the x‐ray‐diffraction measurement, it is found that the short‐range order ofa‐SiNx:H with a basic unit of 4.31 A˚ appears when the nitrogen‐to‐silicon ratio in the film exceeds 0.75 at a substrate temperature of 300 °C. In the infrared‐absorption measurement, the sample was annealed repeatedly at various temperatures (300–800 °C) to identify the molecular unit responsible for each absorption peak. It is found that part of the infrared‐absorption bands between 870 and 1100 cm−1are caused not by the absorption but by the reflection of infrared radiation due to reststrahlen effect. The absorption peak at 840 cm−1is assigned to the isolated N in the Si‐host network, whereas the peak at 885 cm−1is assigned to a local bonding arrangement involving a terminal N‐H group attached to the amorphous Si network, Si‐NH‐Si. The transverse optical phonon of crystalline Si3N4is found to peak at 870 cm−1. Two peaks are observed in photoluminescence experiments when the mole fraction of ammonia in gas phase is smaller than 0.8 indicating the inhomogeneity of thea‐SiNx:H films. The higher‐energy peak is visible in the red.