The tests on running engines reveal that the vibration levels of an engine block are higher at the lower portion than the upper portion.The objective of this paper is to reduce the vibration levels of an engine block by the stiffening plates with minimum weight increase. The analytical modal analyses of an engine block and the stiffening plates made of aluminum and steel are performed by the Finite Element Method (FEM). The experimental modal analyses are also conducted and two results are correlated. In these validated modal model, an engine block and the stiffening plates are combined by the spring elements. The Component Mode Synthesis is used to obtain the natural frequencies and modes of the combined blocks. The iterative analyses are performed with varying thickness of the stiffening plates in order to design the optimum one which reduces the vibration levels of an engine block effectively with minimum weight increase.The running engine tests confirm that the designed stiffening plate reduce the overall vibration levels of an engine block.