To achieve lightweight, low friction and fuel efficient engine, the crankshaft is required to be designed lightweight, small-diameter shaft, long stroke. In this case, vibration of the crankshaft is increased by reduction of shaft stiffness. The conventional way of dealing with this increased vibration used to be to add an inertia mass ring or a double mass damper. Such an approach, however, increases weight, making the balance of weight reduction and vibration reduction less readily achieved.This paper therefore reports on how the main factors causing crankshaft vibration to increase in the shaft with reduced stiffness were clarified. Based on that clarification, efforts were made to reduce crankshaft vibration without increasing the weight of the crankshaft system.Measurement and analysis were used to analyze crankshaft vibration during operation. This showed that the main factors in the increase of vibration in a crankshaft with reduced stiffness are the coupled modes of crankshaft bending and crankshaft damper pulley bending.The relationship between the distribution of eigenvalues in these coupled modes and the stiffness of the crankshaft damper pulley hub was then investigated by carrying out FEM analysis of the crankshaft system.The results showed that when the stiffness of the crankshaft damper pulley hub is increased, the coupled modes are separated, and crankshaft vibration is reduced.Finally, a prototype crankshaft damper pulley hub with increased stiffness was used to conduct verification.The results showed that by changing the shape of the crankshaft damper pulley, the increase in vibration of the reduced-stiffness crankshaft could be recovered by 28% without increasing the weight of the crankshaft system.