We report the magneto-optic (MO) coupling interaction of guided optical waves (GOWs) with magnetostatic waves (MSWs) in MO film waveguides using arbitrarily tilted bias magnetic fields. The universal MO coupledmode equations are obtained and can be applied to the collinear or noncollinear interactions of the GOWs with magnetostatic forward volume wave (MSFVW), magnetostatic backward volume wave (MSBVW) and magnetostatic surface wave (MSSW). As a typical example, the noncollinear diffraction interaction of the GOW with the MSFVW excited by single-element microstrip line transducer in the yttrium-iron-gaet (YIG) film is analysed in detail. For the case of normal magnetization, the calculated plot is consistent with the experimental results in the first passband. By comparison, the diffraction efficiency (DE) can further be improved by optimizing the magnetization direction. The maximum DE gain can reach 5.7dB under the appropriately inclined bias magnetic field at (ψ) = 180° and θ = 9°.