The structural properties of subgrain boundaries (low-angle grain boundaries) in bulk 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 subgrain boundaries were polygonized into (1 (1) over bar 00) directions and most often occurred at the peripheral parts of crystals. The relative misorientation between adjacent subgrains was examined by HRXRD, and it was found that the tilting of the (0 0 0 1) lattice plane occurred at the subgrain boundaries and had a rotation axis parallel to both the boundary plane and the (0 0 0 1) basal plane. Based on these results, we have discussed the cause and mechanism of the formation of subgrain boundaries in bulk SiC crystals.