The excellent transparent in wide region of spectra, nanocomposite SiCxNy:H films were synthesized by RPECVD using hexamethylcyclotrisilazane in mixture of helium and nitrogen in the temperature range of 373-1073 K. The analysis of FTIR, XPS and Raman spectroscopy results showed that low temperature films are hydrogenated silicon oxycarbonitride. There are the formation of chemical bonding between Si, C, N atoms with predominate surrounding of Si atoms by nitrogen atoms and the absence of hydrogenous bonds in high temperature films. C-V and I-V measurements showed that SiCxNy:H films are low-k dielectrics. Thermal annealing of these films leads to their densifying, ordering of structure and increase of nanocrystals' size.