Boron distribution on moving grain boundaries during recrystallization was investigated in Fe-3%Si by means of particle-tracking autoradiography. The experimental results indicated that at the beginning of recrystallization no boron segregation appears on moving boundaries, but as the holding time is increased and recrystallization continues, the boron segregation is intensified and persists with the new boundaries moving forward; as the holding time extends further, the recrystallization is nearly complete and the moving velocity of the boundary decreases, then the boron segregation begins to decrease and disappear. The maximum segregation on the moving boundaries is much higher than the equilibrium segregation estimated by the McLean equation at the same temperature. These results cannot be properly interpreted by the existing theories relating solute segregation to the boundaries.