Abstract: The deflection of crankpin and reciprocating rotation of piston were calculated according to the mathematic model based on the torque transmission theory of crankshaft. The discriminant of synchronous rotation of piston ring with piston was put forward. Moreover, the analytical model about the opposite rotation of the piston ring was established, by taking into account various forces including combustion gas pressure, liquid friction, and reciprocating inertia force, and various moments including rotational friction, rotational inertia, and gas friction moment. It was found that the positive and negative deflections of the crankpin resulted in the reciprocating rotation of the piston in the IC engine work cycles. The peak values of the deflection and rotation were near to the crank angle position in deflagration. Because the clingy moment of the compression ring was less than the rotational resistance moment in a few section of the crank angle, it was unable for the ring to keep synchronism with the piston and hence rotated opposite to the piston. Rotation angle of the second compression ring was larger than that of the first ring. The superposition of the end gaps of the two compression rings occurred when they turned to the same angle position.