<div class="htmlview paragraph">This paper describes a new method for engine design using dynamic optimization and substructure synthesis method. A very important theme in engine design is how to shift the peak of the natural frequency of the vibration mode that causes some noise and vibration problems. This must be resolved by effective modification of structural design.</div> <div class="htmlview paragraph">In order to carry out effectively vibration analysis of a large scaled structure like engine assembly and conduct dynamic optimization with many iterative calculations, we have used substructure synthesis method that devides a whole structure into a number of substructures and solves each substructure.</div> <div class="htmlview paragraph">Vibration analysis of engine assembly (cylinder block, crank shaft, bearing caps and flywheel systems) was carried out by using this substructure synthesis method. And, on the basis of the sensitivity of eigen value of the residual structure that is to be modified (cylinder block and bearing caps this time), the optimization program using Pseudo Least Square Method has been successfully applied to shift the peak of a given natural frequency to a designated frequency range.</div>