Gibbs energy model parameters for Si-Zr-N system were obtained using Calphad approach coupled with ab initio calculations. The enthalpies of formation of α and βSi3N4 in Si-N system and Zr5Si3N end-member in Si-Zr-N system were calculated using density functional theory (DFT). The finite temperature thermodynamic properties were calculated using quasiharmonic approximation (QHA). The computed heat capacities were fitted to appropriate expressions valid down to 0 K. The ab initio thermochemical data obtained in the present work and the experimental thermochemical and constitutional data from literature were used for the thermodynamic optimization of Si-N and Si-Zr-N systems. The calculated phase equilibria and thermodynamic properties are in good agreement with the input data. © 2018 Elsevier Ltd