The reaction abilities of structural units in Fe-C binary melts over a temperature range above the liquidus lines have been evaluated by a thermodynamic model for calculating the mass action concentrations N-i of structural units in Fe-C binary melts based on the atom-molecule coexistence theory (AMCT), i.e., the AMCT-N-i model, through comparing with the predicted activities a(R,i) of both C and Fe by 14 collected models from the literature at four temperatures of 1833, 1873, 1923, and 1973 K. Furthermore, the Raoultian activity coefficient gamma(0)(C) of C in infinitely dilute Fe-C binary melts and the standard molar Gibbs free energy change Delta(sol)G(m,Cdis(1)->|C|w[C]=1.0)(circle minus,%) of dissolved liquid C for forming w([C]) as 1.0 in Fe-C binary melts referred to 1 mass% of C as reference state have also been determined to be valid. The determined activity coefficient ln gamma(C) of C and activity coefficient ln gamma(Fe) of Fe including temperature effect for Fe-C binary melts can be described by a quadratic polynomial function and a cubic polynomial function, respectively.