The present invention relates to an engine structure of a motor cycle. The engine is arranged on the rear of a descending pipe arranged on a cycle frame. The engine comprises a crank case, a cylinder body and a cylinder head, wherein the crank case consists of a left crank case and a right crank case, a crank, a main shaft, an output shaft and an engine fuel pump are arranged in the crank case, and the crank is connected to a piston by a connecting rod; the connecting rod and the piston are arranged in the cylinder body, and the center of the crank does not penetrate through the center of the cylinder body; a cam shaft, a rocker arm, an air inlet, an air outlet and a combustion chamber are arranged on the cylinder head; the cylinder body shifts toward the rear of the cycle body, the central line of the cylinder body shifted is between the center of the crank and the center of the main shaft, and the crank rotates clockwise. Thus, part configuration space before the engine can be effectively increased, and side friction produced when the piston runs is reduced. Meanwhile, the purpose in conveniently arranging an exhaust pipe is achieved.