Amorphous silicon oxygen nitride (aSiOxNy) films are prepared by microwave electron cyclotron resonance chemical vapor deposition (ECR-CVD) method using 10% SiH4 in N2/O2 mixtures. The Fourier transform infrared absorption spectroscopy shows that the main bonding configuration of the films is SiO and SiN. If O2 is input, the dominant composition of the films is SiOx, while it is SiNx at zero O2 flow rate. A blue photoluminescence (PL) band at 460nm produced by 565eV laser excitation is observed at room temperature from these films, and the emission intensity increases with decreasing O2 flow rate. The blue PL for aSiOxNy films, mainly composed of aSiOx, comes from oxygen deficient defects in the SiOx matrix. For aSiOxNy films, mainly composed of aSiNx, its blue PL is due to the electronhole radiative recombination from the defect states in the SiNx matrix.