In order to describe the formation of FeS–MnS–CuS_0.5 inclusion in γ-Fe, the thermodynamic properties of the FeS–MnS–CuS_0.5 system have been investigated at 1473 K. The isothermal section was determined by a chemical equilibration technique. The activities of FeS and CuS_0.5 in the liquid phase were determined by equilibrating with carbon saturated iron or molten copper, and the activity of MnS was evaluated by using the Gibbs-Duhem equation with Schuhmann’s tangent intercept procedure. Finally, the composition of solid steel equilibrated with the FeS–MnS–CuS_0.5 inclusion was estimated using the determined thermodynamic properties.