Abstract: The behaviour of boron segregation on moving grain boundaries during recrj'stallization after high temperature deformation was investigated in a Fe-3% Si alloy by means of boron particle track autoradiography (PTA). Such kind of segregation was observed after 20% deformation at 1000℃, and disappeared as the grain boundaries stopped. Based on the above results a mechanism of dislocation relaxation on grain boundaries is proposed for the segregation on movig boundaries.