The critical temperature (T-c) and boiling temperature (T-b) of working fluids are important selection criteria for the organic Rankine cycle (ORC) system. In this study, the ratio of T-b and T-c (T-br) and the vapor expansion ratio (VER) model based on Claussius-Claperyron equation are introduced to compare and explain their effects on the maximum net output power (W-net,W-max) and VER of ORC. The investigation of 267 working fluids is done at four heat source temperatures (T-5). Maximum vapor enthalpy method is proposed to determine the upper limit of the evaporation temperature, which is the optimization parameter for maximizing the net power output. At low T-5 (423.15 and 473.15 K), the obvious relationships between W-net,W-max and T-c are independent of T-br. Therefore, T-c enables to select working fluids with high W-net,W-max. However, at high T-5 (523.15 and 573.15 K), T-br is essential to exclude working fluids with optimum T-c (0.89 - 0.90T(5)) but low W-net,W-max. Moreover, at a given T-c, high T-b or T-br indicates high VER. Consequently, T-b or T-br is suitable to be used as the second indicator. This paper proposes the optimal combinations of T-c and T-b and the developed composite indicator for selection of working fluids. (C) 2016 Elsevier Ltd. All rights reserved.