Abstract: Based on the phase diagram, the coexistence theory of the metallic melt structure involving compound formation and the regularity of melts containing saturated phase, a calculating model of mass action concentrations for Mn-C melts was formulated. According to the activities (or activity coefficients γ_Mn and γ_C), obtained by the Unified Interaction Parameter Model (UIPM), the thermodynamic parameters △G_Mn3C^⊝ and △G_Mn5C2^⊝ at higher temperatures (1628~1873 K) were optimized by the repeated try and error method. It is testified that △G_Mn23Cn^⊝, △G_Mn15C4^⊝(873~1273 K) and △G_Mn7C3^⊝ (298~1623 K), obtained at lower temperatures, can also be used at the above-mentioned higher temperatures. The calculated N_Mn and N_C agree well with α_Mn and α_C obtained by UIPM. This result shows that the deduced model can reflect the structural reality of Mn-C melts.