Abstract: High power density engine is a development direction of internal combustion engines. Due to its simple structure and twice the power stroke frequency compared with conventional reciprocating piston engines, the opposed rotary piston engine can significantly improve the theoretical power density of an engine, being an ideal power source for small unmanned aerial vehicles and hybrid propulsion systems. In this paper, a mathematical model was developed to investigate the effect of structural parameters of the opposed rotary piston engine on piston motion, cylinder volume variation, and compression ratio. And the boundary conditions, bring the collision between adjacent pistons during their motion, were also analyzed. The results show that the piston linear velocity is the most sensitive element to the eccentricity variation of the elliptical gear pair curve. When the linear velocity exceeds the average value, its variation is direct proportion with eccentricity, and it varies inversely with eccentricity when it is below the average value. Increasing the eccentricity can reduce the clearance volume of the cylinder and increase the cylinder volume corresponding to the bottom dead center, making the compression ratio of the engine be adjusted. Enlarging the angle between the two sides of the piston can reduce the clearance volume of the cylinder. When the eccentricity and the angle between the two sides of the piston vary within a range of −10% and +10%, the corresponding ranges of compression ratio are 5.62~12.04 and 5.01~23.45, respectively. Excessive eccentricity will lead to collisions between adjacent pistons.