SiOxNy barrier films were deposited at low temperature of similar to 23 degrees C using inductively coupled plasma chemical vapor deposition (ICP-CVD). When radio frequency power increased at constant ratios of Ar/SiH4 and O-2/(O-2 + N-2), respectively, of 4.5 and 0.1, the aggregation tendency of nanoparticles slightly decreased, whereas when the ratio of Ar/SiH4 was changed from 4.5 to 41, almost spherical nanoparticles, each of which was isolated without aggregation, were observed. Such a near spherical nanoparticle without aggregation produced highly dense films with enhanced barrier property. However, heavily aggregated nanoparticles produced very porous films with deteriorated barrier property. In other words, the film density and barrier property depended significantly on whether nanoparticles were isolated or aggregated. To investigate the formation mechanism of dense SiOxNy barrier film, a transmission electron microscope grid membrane was exposed for 5 s to capture nanoparticles generated in the gas phase using a shutter above the grid membrane during ICP-CVD and the grid membrane was observed by scanning transmission electron microscope. The formation mechanism of the barrier film could be best explained by non-classical crystallization of CVD films, where nanoparticles formed in the gas phase contribute to the film formation.