Thermodynamic properties constitute part of our general knowledge about physical and chemical properties of nuclear materials, as the solid substance UN1−x(fcc_B1). This is why the thermodynamic modelling of the N–U binary system is performed here from a critical assessment of most of the available experimental information, with one of the most commonly used optimization procedure. Optimized Gibbs energy parameters are given, and a comparison between the calculated and experimental equilibrium phase diagram or thermodynamic properties is presented.