Abstract: In this paper, it is on the basis of "petroleum disperse system theory" that the effects of magnetic treatment on the paraffin grain formation are explained satisfactorily in the light of "DLVO theory".Magnetic field strengthens the Van de Waals force of paraffin particles through the influence on the intermolecular dispersion force and promotes the paraffin particles to gather together, which is why magnetic treatment can prohibit paraffin formation, reduce crude oil's viscidity and change crude oil gel state.