A piston engine has a crank assembly, a piston assembly, a crank case assembly and a cylinder head assembly. The crank assembly has a central axle, two counterweight blocks and a crank axle. The central axle has two connecting segments and two flanges. The flanges are respectively coaxial and eccentric mounted with the connecting segments and each flange has an eccentricity h relative to a corresponding connecting segment. Accordingly, the eccentricity h can make the crank case assembly and the cylinder head assembly vibrate. The vibration directions of the cylinder head assembly and the crank case assembly opposite to the vibration directions of the piston assembly and the crank assembly. Then, the shaking and the vibration of the piston engine can be effectively reducing.