A mathematical model of the inherent frequency of liquid droplets in water-in-oil（W/O）emulsions was developed from the force balance analysis of the liquid droplets with the presence of external electrical field. The demulsification mechanism of high-frequency pulsating electrical field was proposed and it was proved theoretically that there existed an optimum frequency for the best demulsification performance. A prediction model was developed for calculating this optimum frequency. It was also shown that the inherent frequency was dependent on droplet size，surface tension，viscosity，temperature and density. The experiments of the n-octane and the white oil W/O emulsion were carried out to prove the reliability of the prediction model. The theoretical prediction of the mathematical model for optimum frequency agreed well with both the experimental results of the n-octane W/O emulsion and the modified experimental results of the white oil W/O emulsion.