The defect structure of silicon oxynitride films prepared by the ECR Plasma CVD method have been studied by ESR(electron spin resonance) method. The ESR analysis revealed the presence of three paramagnetic defects,that is, Si dangling bonds(g=2.0055 ΔHpp=8 Oe), Si dangling bonds with N atom neighbors(g=2.004, ΔHpp=12 Oe) and E-center(g=2.0016, ΔHpp=4 Oe). These centers strongly depend on the deposition conditions, such as gas flow rate, and microwave powers. The defect density is minimum for the deposition condition of SiON films, under the gas flow O2 /(N2 +02)ratio of 1/3, gas pressure of 8.3x10−4Torr and microwave power of 50OW.