Abstract Amorphous hydrogenated silicon, carbon nitride thin films (a-SiCxNy∶H) are deposited at room temperature by r.f. reactive-sputtering method. Thin film structure and optical, electrical properties are measured and analyzed using IR spectra, optical absorption spectra ［α(λ)］, electron spin resonance (ESR) and electrical conductivity (σ). The dark conductivity σd，optical gap Eopt, ESR spin density Ns as functions of N2 flow γN2 are studied with the H2 flow γH2=12% and N flow γN2 varying between 0—14%. The results show that the thin film structure and properties are obviously modified owing to the co-existence of carbon and nitrogen and suddenly changed when γN2 is about 5%.