The structural properties of low angle grain boundaries in modified-Lely SiC crystals have been studied using high resolution x-ray diffraction (HRXRD) and molten KOH defect selective etching. The defect selective etching revealed that the low angle grain boundaries were polygonized into <1 (1) over bar 00 > directions and more often occurred at the peripheral parts of crystal. The relative misorientation between adjacent subgrains was examined by HRXRD, and it was found that the tilting of the (0001) lattice plane occurred at the low angle grain boundaries and had a rotation axis parallel to the boundary plane. Eased on these results, we have discussed the cause and mechanism of the formation of low angle grain boundaries in modified-Lely SiC crystals.