Electron spin resonance (ESR) spectra in plasma-deposited amorphous SiOx films are investigated as a function of oxygen content, x. For x < 1.5, a broad resonance line around g = 2.005 dominates the ESR spectra. On the other hand, for x > 1.5, several narrow lines become pronounced over the broad line. The narrow lines are assigned to arise from silicon dangling bond centers with . Si drop Si2O, . Si drop SiO2, and . Si drop O-3 configurations. The anisotropic g factors for the . Si drop Si2O and . Si drop SiO2 centers are experimentally determined by means of a curve-fitting analysis. The g factors are also theoretically estimated on the basis of a molecular orbital calculation, and the values qualitatively agree with experimental ones. It is suggested that the broadening of the ESR Line is reduced by the presence of an oxygen atom at the nearest-neighbor site of the trivalent silicon atom.