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In this paper, we introduce the recent progress in research of structural defects in 6H-SiC single crystals. Two kinds of typical structural defects in 6H-SiC single crystals were investigated. Elementary screw dislocations were observed by back-reflection synchrotron radiation topography (BRSRT). The density of elementary screw dislocation was measured to be in the magnitude order of 104/cm2. Based on the strain field of screw dislocation, the synchrotron topographs of an elementary screw dislocation and a screw dislocation with Burgers vector 2c were simulated by ray-tracing method. Simulation results confirmed that almost all the white dots in synchrotron topograph corresponded to elementary screw dislocations. Basal plane bending was detected by high resolution X-ray diffractometry (HRXRD) and transmission synchrotron white-beam x-ray topography (SWBXT). HRXRD reveals that the (0001) Si face is a concave sphere and SWBXT shows that the shapes of Laue spots are different from that of X-ray illumination source. Based on the sphere curvature model of (0001) wafer for 6H-SiC single crystal, the shapes of Laue spots were simulated. The results are in well agreement with the experimental observations.