This paper describes the characteristic and the mechanism of serious bending stress occurred in the crankshaft rear at the whirl occurred. At first, the order tracking analysis is used for the measured bending stress. Then, many different resonances of the frequency and the amplitude occurred, and the curve of each order around resonance engine speed was unique, for example sharply peak, flat peak, and peak of 2 steps. Secondly, this paper described that the resonance frequency of bending stress for forward whirl which occurs in the same direction as crankshaft rotation and for reverse whirl which occurs in the reverse direction as crankshaft rotation increases and decreases with the increasing engine speed. It showed that the whirl occurred in the crankshaft rear-end, as the resonance frequency of measurement result increases and decreases linearly with the increasing engine speed. Then, the stress and an excitation force which is radial force acting on the crank-pin were examined about the phase relation. There was the phenomenon that the phase of stress got behind the phase of excitation force at the forward whirl, and the phase of stress led to the phase of excitation force at the reverse whirl. It occurred by whirl from the results with consideration about the time rate of change in momentum and angular momentum of the crankshaft rear-end to know reasons about it occurred. Therefore it was showed that the phase relation which of the excitation force and crank radial component of the gyroscopic moment and the centrifugal force occurred by whirl affected the bending stiffness of crankshaft rear, generated the unique curve of resonance, and changed amplitude. In addition, to reduce distance of the axial barycenter and mass of the crankshaft rear-end and is considered of value, as reduction handling of the bending vibration at whirl occurred.