According to the coupled-mode theory, the interaction of guided optical waves (GOWs) and magnetostatic waves (MSWs) in magneto-optical waveguide are theoretically studied and the coupled-mode equations are obtained. The angular dependence of GOW diffraction efficiency for anti-Stokes interaction in YIG film are calculated. The theoretical curves have the same characteristics as the experimental results. Only in the case of normal magnetization, the waveguided lights with mode conversion are all diffracted and the diffraction efficiency is equal to the mode-conversion efficiency. Under appropriately inclined bias magnetic field, both mode-conversion efficiency and diffraction efficiency may be increased distinctly in comparison with normal magnetization. After changing the direction of MSW propagation, the angular dependence of DE becomes symmetrical.