An alternate current impedance spectrum was utilized to differentiate the electrical transport process, respectively, in the bulk and the grain boundary of barium tungstate microcrystallines under high pressures up to 20 GPa. For powdered BaWO4 microcrystallines, the grain boundary makes a more remarkable contribution than the bulk to the total resistance. The discontinuities of bulk resistance and relaxation frequency at about 7 and 14 GPa reflect the pressure-induced structural phase transitions of BaWO 4 from scheelite to fergusonite structure and from fergusonite to an unknown disordered structure, respectively. The activation energy of the grain boundary decreases with increasing pressure from 6.9 to 8.9 GPa, indicating that the compression has a negative contribution to the activation energy and the transport of charge carriers through the boundary becomes easier. The activation energy of the bulk also shows a similar phenomenon. In addition, the ascending relaxation frequency of the bulk and grain boundary shows that the polarization process needs much shorter time in the state of three-phase coexistence.