It is experimentally demonstrated that the electric field drives the grainboundary (GB) migration in ceramics, but this has not been interpreted mechanistically. This work develops a phase field model to study the GB migration in alumina (Al 2 O 3 ) and validate through the comparison with previous experiments. Results show that the GBs move to the small grain domain. Under an electric field in the positive bias direction, GB migration is enhanced, whereas the migration to the small grain domain is inhibited under the electric field in the negative bias direction. The enhancement or inhibition effect becomes more pronounced with increasing the electric field. The high negative bias induces decrease in the GB migration velocity even with the migration direction altering. It is revealed that GB migrations are dominated by the competitive effect between the curvature and electric field driving forces, and an analytical expression of the critical electric field is derived.