To express the polycrystalline characteristics of magnetic material in numerical calculation, three-dimensional polycrystalline magnetic field analysis is proposed here. LLG calculation for magnetic domain structure should be used, but it usually has a mesh explosion issue for a lot of crystal grains magnetic material. A local coordinate system is set in each crystal grain to express the crystal magnetic anisotropy, and a whole coordinate system is set in the polycrystalline of magnetic material to express magnetic flux continuity. Coordinate transformation is used so as that the physical quantities derived from the whole coordinate system are expressed as magnetic properties determined by the local coordinate system. Each crystal is assumed to have the same magnetic characteristics as B-H curve of a single crystal. Finite element method is used for its numerical calculation, because magnetic flux density distribution is obtained in the minimum electromagnetic energy condition. Two cases of GO steel sheet of 80 mm square are picked up as 2-crystal grains and 56-crystal grains. Magnetic flux density distribution of the GO steel is measured by needle method for magnetic flux density and H-coil for magnetic field. The proposed calculation result of magnetic flux density distribution expresses well the measured result. By using this technique, the study of the crystal grain shape is carried out. The calculation results are shown that the grains grown horizontally are preferable in order to reduce the iron loss, because magnetic flux density distribution becomes uniform.