Organic Rankine Cycle (ORC) is regarded as a very suitable method to recover the waste heat of internal combustion engines. Engines often operate under variable working conditions, so ORC systems are also under unstable state which means it is important to research the dynamic response. There are many kinds of working fluids and various dynamic response characters are reflected owing to their different properties. The dynamic math models of ORC systems with 14 different working fluids (including water) as waste heat recovery system of a natural gas engine are established by Simulink in this paper. Based on these, their dynamic response characters mainly reflected by rise time, settling time and time constant are compared and analyzed. The results indicate that no matter under the disturbance of working fluid mass flow rate or exhaust temperature, the ORC systems with low temperature working fluids respond faster than those with high temperature working fluids generally. Therein, RC responds the most slowly. Furthermore, the critical temperature can be used to approximately estimate dynamic response speed of ORC with straight-chain alkanes. These provide useful guidance for the selection of working fluids based on the working condition characteristics of different engines and control system design. (C) 2017 Elsevier Ltd. All rights reserved.