Abstract: A single cam reciprocating linked piston type engine utilizes a swing-link linking mechanism to make two pistons arranged at an included angle slide alternately up and down, or utilizes a linking-rod to make two pistons arranged on an identical axis reciprocate in the same direction. Each roller mounted in the piston skirt nestles tightly on the curved surface of a cam mounted on a main shaft, so as to transform reciprocating motion of the pistons into rotation of the main shaft. The engine has eliminated an inner cam and the complicated piston mechanism as compared with a conventional double cam internal combustion engine. It features a simple and compact structure, its output torque is able to be raised twice as much, and a speed reducer matched with the engine is simple in structure. It can replace double cam engines and conventional crankshaft and connecting-rod type engines and is applicable for use as a motor in various fields.