Irradiation creates intrinsic defects such as vacancies, interstitials and antisites in SiC. In addition, the most stable intrinsic defect is the carbon antisite which can be formed even during growth of SiC. Ab initio theoretical calculations have shown that the carbon antisites can migrate in SiC and can form aggregates with the silicon and carbon antisites. We will show that the former one, i.e. antisite-pair can be responsible for the D, photoluminescence spectrum, while the latter one may be the core of the D-II defect. It has been earlier found that the carbon interstitials has relatively low barrier energy for migration. These mobile species can form aggregates with themselves and. with the carbon antisites. Indeed, we have found that the formation of carbon clusters is very feasible process, and one form of these clusters may be responsible for the P,Q,R,S,TU photoluminescence lines. The carbon di-interstitials form several metastable defects in hexagonal SiC with acceptor states, and they might be the origin of the thermally extremely stable deep level transient spectroscopy centers.