According to the coupled-mode theory for guided optical waves, the mechanism of magneto-optical four-wave mixing (MO-FWM) existing in a fiber with magneto-optic effect and nonlinear effect both serving as perturbation is analyzed. The influence of applied magnetic field on the polarization dependency of FWM is measured, and the experimental results are in agreement with the theoretical analyses. It is shown that the magnetic control function of FWM can be identified easily for the incident orthogonal states of polarization. The FWM efficiency is dependent on the MO coupling coefficient in the oscillation form and can be further improved under an appropriate bias magnetic field. The MO-FWM effect can also be applied to magnetic field sensing, and the operating range is determined by the Verdet constant of highly nonlinear fiber.