This study analyzes the system performance of a 50 kW ORC system subject to influence of various working fluids. A dimensionless "figure of merit" combining the Jakob number, condensing temperature, and evaporation temperature is proposed for quantitatively screening working fluid as far as thermal efficiency is concerned. The thermal efficiency normally decreases with the rise of figure of merit, and the predictive ability of the proposed figure of merit is not only applicable to the present eighteen working fluids but is also in line with some existing literatures. Analysis of the typical ORC heat exchangers indicates that the dominant thermal resistance in the shell-and-tube condenser is on the shell side. Similarly, the dominant resistance is also on the refrigerant side for the plate evaporator. However, there is a huge difference of thermal resistance amid working fluid and water side in the preheating zone. Conversely, only a minor difference exists in the evaporation region. The extremely uneven resistance distribution in the plate heat exchanger can be resolved via an additional preheater having significant augmentation in the working fluid.