The electromagnetic theory is used to characterize the polarization processes in glasses. At low B2O3 content, the electric field treatment inhibits the phase separation of the glass. But, at low P2O5 content, electric field treatment promotes the process. The analysis shows that the free energy may be reduced or enhanced by the electric field depending on the ratio of the dielectric constants of the old and new phases. This means that the electric field can either stimulate or inhibit the nucleation process. The theory is in excellent agreement with the experimental results. And, we may extend it to the phase transformation of all dielectric materials applying an electric field. These results suggest a possible method for the preparation of nanophase glass-ceramics.