A thermodynamic model for calculating the mass action concentrations Ni of structural units in Fe-Al binary melts based on the atom-molecule coexistence theory, i. e., AMCT-N-i model, has been developed and verified to be valid by comparing with reported activities a(R,i) of both Al and Fe relative to pure liquid as standard state in Fe-Al binary melts over a temperature range from 1823 to 1973 K (1550 to 1700 degrees C). Furthermore, Raoultian activity coefficients gamma(0)(i) of both Al and Fe in the Fe-rich corner or Al-rich corner of Fe-Al binary melts as well as the standard molar Gibbs-free energy changes Delta(sol)G(m,i(l)->[i][%i]=1.0) (Theta,%)of dissolved Al or Fe for forming [% Al] or [% Fe] as 1.0 in Fe-Al binary melts have also been determined by the developed AMCT-Ni model and verified to be accurate. The reported activities a(R,i) of both Al and Fe from the literature can be well reproduced by calculated mass action concentrations Ni of free Al and free Fe in Fe-Al binary melts. A small effect of changing temperature from 1823 to 1973 K (1550 to 1700 degrees C) on reaction abilities of both Al and Fe from the available literature is also confirmed by calculated mass action concentrations Ni of free Al and free Fe in Fe-Al binary melts. The obtained activity coefficients gamma(i) of both Al and Fe in Fe-Al binary melts can be described by a quadratic polynomial function and a cubic polynomial function, respectively. Furthermore, accurate expressions of Raoultian activity coefficients gamma(0)(i) of both Al and Fe in Fe-rich corner or Al-rich corner of Fe-Al binary melts are also obtained as ln gamma(0)(Al)=-9,646.5/T+2.196 and ln gamma(0)(Fe)=-6,799.1/T-0.01367, respectively. In addition, expressions of the first-order activity interaction coefficients epsilon(i)(i) or e(i)(i) or h(i)(i) of both Al and Fe coupled with three activity coefficients gamma(i) or f(%,i) or f(H,i) relative to three standard states are also obtained from the developed AMCT-N-i model for Fe-Al binary melts.