This paper presents a theoretical model on the payback period of a subcritical ORC (organic Rankine cycle) system for recovering low-grade waste heat of flue gas. Based on the minimum payback period principle, a detailed internal parameter optimization is carried out. Furthermore, the influences of external parameters on the payback period are analyzed and a new criterion of screening working fluids is proposed. Results show that the payback period of the ORC system decreases before increasing with the increase of evaporation temperature, condensation temperature, and the pinch point temperature differences in the evaporator and condenser. The minimum payback period of the ORC system decreases monotonously with the increase of the inlet temperature, mass flow rate of the flue gas, and the electricity price. When the minimum payback period and the net power output are used as indicators of selecting working fluids of the ORC system, this work identifies a specific working fluid for a given inlet flue gas temperature. We demonstrate this process by showing that R236a, R245fa, and R113 should be selected as the working fluids when the inlet temperatures of the flue gas are around 150 degrees C, 200 degrees C and 250 degrees C, respectively. (C) 2015 Elsevier Ltd. All rights reserved.