Abstract: In this paper, comparative experiments with and without diurnal variation of the solar radiation are made by use of a 5 layer primitive equation air-sea coupled model system without oceanic currents in a zonal domain between 60°S and 60°N. The results show that the diurnal variation of the solar radiation does not have very large influence on the mean monsoon system in the quasi-equilibrium state, the main influences may come from the land-sea and the topography distributions. However, its inclusion into the model does improve the simulations of the monsoon systems at the upper and the lower levels. The diurnal variation of the solar radiation influences the simulated precipitation pattern quite a lot. In the experiment without the diurnal variation of the solar radiation, the precipitation amount over the land areas is greatly reduced while it is increased over the areas along the coasts. The changes of the soil temperature and moisture fields are well corresponding to the changes of precipitation. The areas with increased precipitation and the areas with decreased precipitation are distributed in a wavelike form. As the monsoon development is concerned it is shown that the diurnal variation of the solar radiation can speed up the development at the early stage. Therefore, the inclusion of the diurnal variation of the solar radiation can make the summer monsoon reach the quasi-equilibrium state earlier.