Thermodynamic descriptions of the ternary Fe-B-Si system and its binary sub-system, B-Si, are developed in the context of a new Fe-B-X (X = Cr, Ni, Mn, V, Si, Ti, C) database. The thermodynamic parameters of the other binary sub-systems, Fe-Si and Fe-B, are taken from earlier assessments. Experimental thermodynamic and phase equilibrium data available in the literature has been used for the optimization of the thermodynamic parameters of the Fe-B-Si and B-Si systems. The solution phases are described using substitutional solution model and the compounds (silicides and borides) are treated as stoichiometric phases. The calculated and experimental thermodynamic and phase equilibrium data were found to be in good agreement.