Abstract: The sub-lattice model proposed by Bjorn Uhrenius, which was used to describe thermodynamic properties of Fe-C-based ternary systems, has been extended and applied to Fe-C-bssed quaternary alloy systems with the Gibbs energy expressions also being given. The phase equilibria between austenite and carbide in Fe-Cr-Ni-C quaternary system have been calculated based on the calculation of thermodynamic equilibria in Fe-Cr-C ternary system done by Bjorn Uhrenius, in combination of the interaction parameters in Cr-Ni binary system given by Nishizawa.