Abstract: An exact theory for an optical acousto-optic phase modulator with a ZnO coating (OF-AOPM) has been developed, which is made by considering only the radial strain induced by acoustic wave in the resonator.It has been shown that radial strain alone gives strong modulation in the vicinity of a modulating frequency close to half-wave resonance of the Piezoelectric ZnO Coating.At frequency far from this region, not only radial strain but also axial strain may be counted on for optical modulation.In this paper, phase modulation in OFPM for the entire frequency region is presented by considering both axial and radial strains.The theoretical calculation shows good agreement with the experimental results.