Abstract: By using the method of shape optimization, the rigidity of 4100QB diesel cylinder block was raised based on HyperWorks/Optistruct software, meanwhile the sensitivity analysis of the cylinder block was carried out to illustrate the design variable effects on response function. There were three kinds of forces in optimizing course applied in various load steps: maximum combustion pressure, maximum piston lateral force and maximum lateral force on the main bearing. The objective of optimization was the maximum sum of modal frequency related to the modal shape of crankcase. Twelve structure dimensions of the crankcase were adopted as design variables and the constraint functions were strain energy of the cylinder block, point displacement and weight of the cylinder block. The results showed that strain energy and frequency of the cylinder block had been improved at various degrees. This optimizing method provides certain reference function for the optimizing of other kinds of cylinder blocks.