Field assisted sintering (FAST) has been demonstrated for multiple ceramic materials as a promising sintering technique in shortening consolidation time and lowering sintering temperatures. Mechanistically the interplay between electric field and the polycrystalline material is a complicated process and the governing factor for rapid densification during FAST is still under debate. In this work, we approached this problem from computational modeling in the model system of rutile TiO2. We separately quantified the effect of Joule heating and non-contact electric field on ion diffusion both in bulk and at grain boundaries. In bulk, we predicted the equilibrium defect concentrations at various pressures and temperatures from first-principles calculations. Berry phase calculation with electric field up to 8 MV/cm shows that electric field has negligible effect on both defect formation energy and migration barrier in bulk TiO2. For grain boundaries, we studied titanium diffusion by classical force field molecular dynamics. Ti diffusion is orders-of-magnitude faster close to the grain boundary. We also demonstrated the defect concentration change in space charge layer formed at grain boundaries with external bias. These results indicate that the local atomic structure and space charge layer profile of grain boundaries are important in understanding electric field effect on polycrystalline materials.