Electromigration in beta-Sn has shown a 10% drop of resistance due to the anisotropic properties of the material. The drop was proposed due to reorientation of grains to reduce the resistance. The driving force as well as the atomic mechanism of grain rotation under electromigration has been considered in this letter. We propose that the anisotropic resistivity causes the divergence of the vacancy fluxes at the grain boundaries and induces the vacancy fluxes to/from the free surface along the grain boundary. The vacancy gradients along the grain boundaries correspond to the gradients of stress. The opposite sign of the stress along grain boundaries generates a torque which leads to rotation of the grain by grain boundary diffusion or creep. The rate of rotation estimated on the base of the model seems to agree well with the observed experimental data.