The equilibrium compositions of a thermite reaction system (Fe2O3+Cr2O3+CrO3+NiO+O-2+N-2+TiO2+C+S+CaO+Al), which was used to prepare Ni-based alloys, were predicted by means of the free energy minimization method. The effects of the reactant contents on the equilibrium compositions and the adiabatic temperature of the thermite reaction system were analyzed. The microstructure of the Ni-based alloys was investigated. The thermodynamic analysis indicated that the products of the thermite reaction system contained primarily Al2O3, Fe, Ni, Cr, Ni3Al and NiAl, due to impurities C, N-2 and S in the raw materials, there could also exist TiC, Cr7C3, CrN, CaS and TiN in the equilibrium products. The amount of Ni3Al and NiAl in the products increased with increase of the aluminum content in the starting materials. The microstructure investigation showed that the Ni-based alloys consisted of austenite, ferrite, Ni3Al and NiAl. The existence of Ni3Al and NiAl in the Ni-based alloys reconciled with the predictions of thermodynamic analysis, suggesting the free energy minimization method is useful in designing advanced materials with multiple components by thermite reaction technique.