Carbon is an important defect in silicon (Si) as it can interact with intrinsic point defects and affect the operation of devices. In heavily irradiated Si containing carbon the initially produced carbon interstitial - carbon substitutional (CiCs) defect can associate with self-interstitials (SiI’s) to form, in the course of irradiation, the CiCs(SiI) defect and further to form larger complexes namely CiCs(SiI)n defects by the sequential trapping of self-interstitials defects. In the present study, we use density functional theory to clarify the structure and energetics of the CiCs(SiI)n defects. Here we report that the lowest energy CiCs(SiI) and CiCs(SiI)2 defects are strongly bound with -2.77 eV and -5.30 eV, respectively.