The ORC (organic Rankine cycle) system is one of the most effective approaches for recovering energy from low grade heat sources like geothermal water. This paper describes the screen of fluids for geothermal ORC system. This study analyzed the influence of the working fluid properties of HC (hydro carbon) and HFC (hydro fluorine carbon) working fluids on the system performance and relates the properties to the molecular structures. HC and HFC working fluids were adopted. A theoretical ORC model was used to optimize the evaporating temperature to maximize the work output for each working fluid. The optimal working fluids are given for heat sources at 383.15 K, 403.15 K and 423.15 K. The results show that for a specific source temperature, the optimized evaporating temperatures for all the working fluids are almost the same. Based on the influence of working fluid property on system performance, two indicators are given for screening working fluids. Fluids like R32, R134a and propylene with GWP (global warming potential) value less than 1500 provide better performance than others, by extracting more energy from the heat source. (C) 2013 Elsevier Ltd. All rights reserved.