Advances in diesel generating field in recent years have resulted in high turbocharged diesel engines and compact generators. However, it raised new problem, i. e. poor ability against load change which causes large speed deviation, compared to conventional electric power units. In order to design proper electric power units, the practical means for studying performance of diesel engines under transient conditions, are required. In this paper, a mathematical model for four-stroke diesel engines coupled with generator developed to meet the above mentioned requirement and comparisons of calculations by means of a Hybrid computer with measurements on the test bed, are described. The mathematical model consists of engine, turbocharger, speed governor and generator, further, for the purpose of general application, all variables are converted into non-dimensional values divided by the valures at rated output. Reasonable agreement between calculations and measurements using a 480 kw turbocharged diesel generator is obtained, which proves the validity of the similation program. And using the simulation program, a comparison of the response ability between mechanical-hydraulic and electric-hydraulic governors is also described.