The internal and external exergy efficiencies are adopted to analyze the impact of working fluids on the performance of the organic Rankine cycle, and a simplified internal exergy efficiency model is proposed to indicate this impact. The calculation results show that the thermophysical properties of the working fluid have little impact on internal exergy efficiency, but they do play an important role in determining external exergy efficiency. Under the same heat source, the internal exergy efficiency of the ORC (organic Rankine cycle) will increase for all working fluids, whereas the characteristics of the external exergy efficiency exhibit parabolic-like curves with the increase of evaporation temperature. By taking exergy efficiency as an objective function, an optimal analysis based on a genetic algorithm is conducted to illuminate the impact of working fluids on internal and external exergy efficiencies. The optimization results show that the selection of working fluids depends greatly on optimal evaporation temperature. Working fluids with lower critical temperature lead to a higher optimal evaporation temperature, which results in higher overall exergy efficiency. Therefore, the present study on selecting working fluids for organic Rankine cycle systems may have potential application in low grade waste heat recovery systems. •The internal and external exergy efficiencies are studied.•The internal exergy efficiency model is proposed and compared with theoretical data.•The thermophysical properties of the working fluid have little impact on internal exergy efficiency.•The working fluid's properties play an important role in determining external exergy efficiency.•Low Tcr working fluid is preferable under optimal operation condition of ORC.