Thermodynamic optimization of the Si-N, Si-C, C-N and Si-C-N systems were performed using the CALculation of PHAse Diagrams (CALPHAD) method based on critical evaluation of all available experimental data. The liquid and solid solutions within the Si-C-N system were described using the Modified Quasichemical Model and Compound Energy Formalism, respectively. The Gibbs energies of β-Si₃N₄ and β-SiC were carefully optimized to resolve inconsistencies among existing thermodynamic properties and phase equilibria data. The solubilities of C and N in both solid and liquid Si solutions were critically optimized. Besides, the solubility of N in β-SiC to form a narrow homogeneity range of β-Si(C,N) solid solution is considered for the first time in the present thermodynamic modeling. Based on the present thermodynamic database, experimentally unexplored phase diagrams and thermodynamic properties within the Si-C-N alloys were predicted. As an application of the database, the influences of various operation conditions on high purity SiC production using sublimation process were calculated.