To reduce diesel engine noise that is induced mainly by main bearing impact forces, two types of low noise concepts of basic crankcase structures were studied.One is the “Isolated Skirt Type”, which has the feature to suppress vibrations of engine surface by separating the crankcase skirt from the main bearing caps.The other is the “Bed Plate Type”, which embodies the feature to suppress vibrations by stiffening the lower part of crankcase by adopting a bed plate design.Dynamic characteristics of both prototypes were investigated by means of experimental modal testings such as double pulse laser holography system and impulsive hydraulic excitation test rig which simulates the exciting force of combustion gas pressure in cylinder.As the result of many experimental tests, it was concluded that the “Bed Plate Type” was advantageous over the “Isolated Skirt Type” in terms of engine noise reduction.During the second development stage of a low noise engine, the “Bed Plate Type” was optimized by FE dynamic calculations according to the following steps, mainly for ensuring weight reduction. (1) Crankcase vibration was predicted by the simulation of bearing impacts. (2) Parametric studies with respect to various modifications were carried out to assess the noise reduction effects per weight increase. Thus, a low-noise and light-weight crankcase has been developed. It achieved 3.5 dB(A) reduction in crankcase noise and 1.5 dB(A) reduction in total engine noise by only modifying crankcase, without any weight increase. This crankcase will provide a good basis for low noise engines.