Silicon nitride (SiNx) films applicable to microelectronics are deposited by photoassisted chemical vapor deposition (CVD) with a low-pressure Hg lamp (185 nm). We describe the deposition mechanism of SiNx in the photo-CVD based on analyses of film structures and transient mass spectroscopy. NH3 is decomposed into NH2 and H by UV-light irradiation. SiH4 is decomposed through the reaction with NH2 or H. SiNx films are formed through an intermediate species formed in gas phase. The formation of the intermediate species is controlled by the decomposition of NH3. At a substrate temperature of 350 degrees C, the deposition rate is controlled by the amount of the intermediate species with higher mass (amu 77, Si2NH7), acid the films are pdlymeric solids. At 500 degrees C, chemical species are deposited on the surface before they grow into the intermediate species with the high mass (amu 77), and the film structure becomes stoichiometric SiNx.