Abstract Requirements of recycling low temperature waste heat energy from internal combustion engines drive the developments of excellent performance expanders with high compactness which significantly affects the applications of waste heat recovery systems to on-road vehicles. In the present study, an opposed rotary piston expander was proposed for the practical utilisations on a small-scale Organic Rankine Cycle (ORC) system, aiming at recycling the waste heat energy from internal combustion engines of on-road vehicles. The opposed rotary piston expander had a cyclic period of 180° crank angle (CA), four intake ports and two discharge ports. In order to investigate the expander performance, 3D numerical simulations were conducted under various scenarios whose boundary conditions were among the frequently reported thermodynamic states in ORC systems; additionally, these scenarios were around the design operation point of the expander. Intake and discharge characteristics, in-cylinder pressure evolutions, in-cylinder fluid flow, and P-V diagrams were analysed; further, volumetric efficiency, power output and adiabatic efficiency were calculated using the simulation results, and were compared to various types of expanders. Each two opposed cylinders had the same evolutions of cylinder volume, fluid mass, in-cylinder pressure, and temperature during operation. Maximum fluid flow rate in the intake process increased with intake pressure and rotation speed; in addition, the in-cylinder pressure reached the maximum value in a short time after the intake ports opened. However, high rotation speed also led to a drop of in-cylinder pressure (expansion process), volumetric efficiency, and adiabatic efficiency compared to low speed condition.