The survey in this paper attempted to provide a comprehensive analysis of thermophysical properties of transmission medium in hydraulic retarder. Its influence, whether quantitative or qualitative, on the internal flow structure was investigated experimentally and numerically. The thermophysical parameters of transmission medium were firstly measured on the test bench to be mathematically formulated for numerical simulation. After that, the Large Eddy Simulation (LES) with Dynamic Subgrid Model (DSL) method was coherently applied to conduct the numerical simulation for the hydraulic retarder and the formulated thermophysical properties were defined by user-defined function (UDF) approach. The investigation obtained some useful discoveries. The density, viscosity and thermal conductivity of transmission medium decreased with the temperature increasing, while the specific heat increased. The decrease in density reduced the pressure and impact resulting in a decrease in braking torque. The decrease in viscosity, accompanied by a decrease in the viscidity between fluid and wall as well as an increase in fluid velocity, lead to the increase for braking performance. The role of specific heat was indirectly reflected by the effects of density and viscosity through affecting the temperature in working chamber. The thermal conductivity had no obvious influence on the braking performance. So it could be concluded that the density and viscosity were the key factors that affected the braking performance, and the specific heat and thermal conductivity did not directly affect the performance and flow field structure of hydraulic retarder. Therefore, there were two issues presented during developing a hydraulic retarder. The dynamic thermophysical properties with temperature in numerical simulation should be employed to avoid errors. Moreover, the transmission medium with larger density and good viscosity temperature characteristics could ensure a better performance for hydraulic retarder. (C) 2018 Elsevier Ltd. All rights reserved.