An organic Rankine cycle coupling free piston compressor (ORC-FPC) system has been proposed, which is used in recovering the waste heat of exhaust gas from the stationary compressed nature gas (CNG) fueled internal combustion compressor. The free piston compressor functions as an expander in ORC and operates reciprocally to compress natural gas in compression cylinders to demanded pressure. After capturing the waste heat available and turning into vapor in evaporator, the working fluid R245ca can provide power to drive the free piston moving reciprocally in expander. The model of ORC-FPC built up in the GT-suite ver. 7.0 assists evaluating performances of this system under different operating conditions. In this paper, the operating condition includes two aspects: thermodynamic state of working fluid and input power. The purpose of simulation based on the model is to specify appropriate thermodynamic states of working fluid which yield high value of eta (the ratio of work produced by the power piston to enthalpy reduction of working fluid in the power cylinder) and k (the ratio of output CNG's mass to enthalpy reduction of working fluid in the power cylinder) value. Performances of the ORC-FPC under different input power, which determined by the operating frequency and injection quantity of the heated working fluid, have also been evaluated. Results show that when the heated working fluid is at 11.5 bar and 383 K, the system achieves better performances than other thermodynamic states, of which k is 601.1 mg/kJ and eta is 44.3%. Based on the optimum thermodynamic state and the principle of obtaining the maximum k, the specific input power determined by (m) over dot (332 g/cycle) and T-cyl (1.2 s/cycle) can make the system achieve better performances, of which k is 648.9 mg/kJ and eta is 42.8%. Performances prediction in the real system based on the simulation results has been evaluated in this paper for its intended application. Through calculation and rational analysis, the compressing ability of ORC-FPC in real-world can reach 9.04 L/s and its actual size can be increased based on the required compressing capability of the final application. (C) 2014 Elsevier Ltd. All rights reserved.