The effects of the grain size on the magnetic properties in extra-low carbon steel after strain annealing were investigated. Two kinds of sample were prepared. One is the annealed sheet, which was annealed at $670^{circ}C$ and $850^{circ}C$ for various time periods after cold rolling. The other is the strain annealed sheet, which was temper rolled by 0.4% and subsequently strain annealed at the temperature ranging between $670^{circ}C$ and $850^{circ}C$ for various time periods. The grains after strain annealing became more coarse than those after primary annealing. The grains were coarsened due to the strain induced grain boundary migration (SIGM). It was found that the permeability tended to be increased and coercivity tended to be decreased with the increase of grain size. The optimum magnetic properties was achieved after strain annealing at $850^{circ}C$ for 30 minites. Under this condition, the coercivity was measured to be 0.6 and the permeability was measured up to be 13000.