Organic Rankine cycle (ORC) is one of the most promising methods for converting medium-low temperature thermal energy into power. Many studies have been performed to optimize the operating conditions, select proper working fluids, design efficient expansion machines, etc. However, as the major power-consuming component in ORC system, the circulating pump is rarely studied. Most of theoretical studies only assume a constant value of isentropic efficiency for the compression process, but this value varies with different studies. A small-scale ORC system is built in the present work to test the performance of a diaphragm pump, and the working fluids of R245fa, R123, R152a and R600a are tested under various conditions. The isentropic efficiencies of diaphragm pump for compressing those working fluids are between 57.22% and 93.51%. The results show that volume flow rate and pressure difference have great influence on isentropic efficiency, and the isentropic efficiency increases with the volume flow rate and pressure difference. Under the condition of same volume flow rate and pressure difference, the isentropic efficiency conforms to the order of eta(R245fa) > eta(R123) > eta(R600a) > eta(R152a). The analyses about the variation trend of isentropic efficiency are made from the perspectives of physical properties of working fluid and the driving mode of diaphragm pump, which prove that the hitting velocity of diaphragm to the liquid is the main cause for irreversible enthalpy increment. A novel parameter alpha(v)/rho c(p), is proposed to identify the influence of physical properties of working fluid on the performance of pump. The experimental results show that under same volume flow rate and pressure difference, the isentropic efficiency of pump decreases with the increment of alpha(v)/rho c(p), of different working fluids. (c) 2017 Elsevier Ltd. All rights reserved.