Abstract: A rotary modulation engine enabling modulation of rotary pistons' rotational speeds is introduced. The rotary modulation engine utilizes four elliptical gears to drive two rotors that overlap each other with rotary pistons thereon alternating one another and rotating in the same direction. The elliptical gears also modulate the relative rotational speeds of these rotary pistons to thereby complete compression, power, exhaust and intake strokes four times in one revolution of a power output shaft of the engine. The rotary modulation engine with the above arrangements has smaller volume and improved efficiency as compared to the conventional four-stroke engine that requires two revolutions of the engine's crankshaft to complete four strokes. The rotary modulation engine can be applied to cars, ships, power generators and the like, and has reduced number of parts, volume and manufacturing cost while provides effectively upgraded operation efficiency.