The crystal grain shape of GO sheet is considered by three-dimensional polycrystalline magnetic field analysis in which grains grown horizontally and grains grown vertically are compared. Because the grains grown vertically have more grains grown nearly parallel to the direction of the magnetic field than the other ones, they have uniform magnetic flux density distribution and then small iron loss. Therefore, the grains grown vertically are preferable to the grains grown horizontally in order to reduce the iron loss.