Mechanism of electric demulsification and the coalescence behavior of internal water drops in oil under electric field was discussed. On the basis of this, a new idea of critical electric field intensity was proposed. Influence of various conditions on electric demulsification was examined experimentally using a batch DC electric demulsifier, The idea of critical electric field intensity proposed in this paper could explain some of the experimental phenomena very well. Besides, a mathematical description for phase separation of emulsion in electric field had been derived and proved to be in good agreement with experimental data.