This paper describes based on the mechanism of whirl resonance about the characteristic of serious bending stress which occurred in the crankshaft rear. The following steps to prove that the bending stress is caused by whirl resonance of the crankshaft rear end. At first, the authors showed that the resonance frequency of bending stress for forward whirl which takes place in the same direction as crankshaft rotation increases with the increasing engine speed, and the resonance frequency of bending stress for reverse whirl which takes place in the reverse direction as crankshaft rotation decreases with the increasing engine speed. Secondly, it was found out that there are two groups, the correlation between the resonance frequency and the resonance engine speed of the measured bending stresses in the crankshaft rear. One is the group that the resonance frequency increases with the increasing engine speed. The other is the group that the resonance frequency decreases with the increasing engine speed. Accordingly, it made sure that the measured bending stress is caused by the whirl resonance. Then, it was compared second-order phase angles of the radial force acting on the No.4 crank-pin which is the excitation force for whirl resonance and those of the rotational fluctuation of flywheel which causes the fluctuation of gyroscopic moment, as the second-order bending stress was the largest. According to the comparison result, the bending stress caused by the gyroscopic moment is added to the bending stress caused by the whirl resonance. Finally, this paper showed about the preventing method of whirl resonance based on the result with calculated three-dimensional vibration of the crankshaft rear. And then, the whirl resonance is large impacted by the mass of flywheel and clutch.