Abstract As one of the most promising methods to convert medium- and low-temperature heat into power, organic Rankine cycle (ORC) has been widely studied. Working fluid, which plays the most important role in ORC, is the root of the huge gap on energy-efficiency between the actual cycle and ideal cycle. This paper presents the limiting thermal efficiency and limiting thermodynamics perfection of simple organic Rankine cycle (S-ORC) and regenerative organic Rankine cycle (R-ORC) in subcritical region to quantitatively describe the role of the pure working fluids. The expressions of limiting thermal efficiency and limiting thermodynamics perfection of S-ORC and R-ORC are derived respectively. 20 working fluids are employed in S-ORC and 10 working fluids are employed in R-ORC to demonstrate the effects of working fluids and operating conditions on limiting thermal efficiency and limiting thermodynamics perfection. The limiting thermal efficiency of S-ORC increases with the increase of the slope of working fluid saturated liquid line and latent heat of vaporization. The limiting thermal efficiency of R-ORC increases with the increase of the slope of working fluid saturated liquid line and latent heat of vaporization and the decrease of the slope of working fluid saturated gas line and specific heat capacity of superheat gas at constant pressure. According to the results of limiting thermal efficiency, the maps for S-ORC and R-ORC which might guide the selection of working fluids for different operating temperature are provided as well.