In the internal combustion engine (ICE), about 40% of fuel energy is released into the atmosphere through waste gas. To recovery the energy, the Organic Rankine Cycle (ORC) has been widely used, and lots of previous studies have selected the rotating turbine as the expander of ORC. However, the rotating turbine has disadvantages of high manufacturing cost and narrow applicable range. For the above reasons, a free piston with constant force output which functions as expander in ORC is proposed to recover the waste energy of exhaust gas from internal combustion engine (ICE). In the system, the free piston with constant force output operates reciprocally to output work under the driving of working fluid R245ca, which absorbs heat from waste gas and provides vapor power. As there is approximately 20% of work is wasted to overcome friction between power piston and cylinder in traditional free piston structure, setting clearance between power piston and cylinder can eliminate friction loss in theory, but results in a negative impact of increasing the extent of working fluid's leakage. Therefore, studying the effect of cylinder clearance on the performances of ORC-FP becomes the study aim of this paper.The model of ORC-FP is built up in the GT-suite which assists studying influence on this system under different clearance. To ensure the model's convergence and steady during simulation, same damping coefficient has been set in all different clearances. Meanwhile, the percent of damping loss can nearly represent the percent of friction loss in the total W because that there is no available parameter to set for simulating the friction condition in the GT software. Results show that W (work produced by the power piston) and Wnet (which is equal to the work of overcoming constant forces in number) decrease gradually as the clearance increase from 0.00001mm to 0.3mm, but decrease dramatically when the clearance varies from 0.35mm to 0.5mm. In conclusions, at the premise of same damping coefficient, Wnet decrease about 9.5% at 0.3mm-clearance comparing with the initial condition without clearance. Nevertheless, friction loss (about 20% of W) will vanish based on theoretical analysis and can make up the loss of Wnet, as a result, Wnet of ORC-FP can increase about 10% at 0.3mm-clearance.With the improvement of the processing technology, the increasing proportion of Wnet can be improved to a higher level.