Abstract: Gibbs free energy change (ΔG) can measure the maximum non-volume work done by the system to the outside, and provide a criterion for the study of the thermodynamic performance of organic Rankine cycle (ORC) with zeotropic mixture working fluids and the determination of the optimal component ratio of mixture working fluids. In this paper, ΔG was introduced into the general thermodynamic model of the ORC, and the relationships between power, heat and ΔG were derived. The zeotropic mixture working fluids of R245fa/ R227ea were used for simulation calculation. Aspen Plus software was used to simulate and calculate the ΔG of each process.The functional capacity of the zeotropic mixture working fluids could be measured, the best component ratio of zeotropic mixture working fluids could be determined, and the heat absorption and net output power could be calculated. The comparison and analysis of the simulation results and the references show that the change trends of net output power and thermal efficiency calculated by the deduced ΔG equations are consistent with the reference, and calculation errors are within the allowable range. The maximum value of ΔG_e in the endothermic process can be used as the basis for determining the component ratio of zeotropic mixture working fluids with the maximum thermal efficiency; the maximum value of ΔG_t in the expansion process can be used as the basis for selecting the component ratio of zeotropic mixture working fluids with the maximum output power. When the mass fraction of R245fa is 0.589, ΔG_e achieves the maximum, the maximum thermal efficiency is 7.514%; when the mass fraction of R245fa is 0.493, ΔG_t achieves the maximum, and the maximum net output power is 4.176 kW.