Under constant current electromigration, white tin (β-Sn) exhibited a resistance drop of up to 10%. It has a body-center tetragonal (BCT) structure, and the resistivity along the a and b axes is 35% smaller than that along the c axis. Microstructure evolution under electromigration could be responsible for the resistance drop. Synchrotron radiation white beam x-ray microdiffraction was used to study this evolution. Both stress and grain orientation was studied. Grain-by-grain analysis was obtained from the diffracted Laue patterns about the changes of grain orientation during electromigration testing in ex-situ and in-situ samples. We observed that high resistance grains re-orient with respect to the neighboring low resistance grains, most likely by grain rotation of the latter. A different mechanism of microstructure evolution under electromigration from the normal grain growth is proposed and discussed.