The activity and solubility of carbon in liquid silicon were calculated using the ideal interstitial solution model. The central atoms model of binary interstitial solution was introduced into the study of carbon solubility in silicon for the first time. The temperature dependence of the carbon activity coefficient and solubility in liquid silicon was presented. Good agreement with the reported data in the literature proves the validity of central atoms model in the thermodynamic studies of a Si-C binary interstitial solution.