An atmospheric general circulation model with a 20-km grid size (TL959) was developed for climate modeling (20-km model). The model has 60 vertical levels (12 levels below 850 hPa), with the top at 0.1 hPa. Time-integration was accelerated by a two-time level semi-Lagragian scheme. In order to represent subtropical marine stratocumulus off the west coasts of the continents, a new stratocumulus parameterization scheme has been introduced following a simple and classical one proposed by Slingo (1987) with some modifications (Kawai and Inoue 2006). Cloud is formed in the model when there is inversion at the top of boundary layer and mixing layer is formed near the sea surface. Using coarser resolution models in vertical and horizontal direction compared with the 20-km model, it was confirmed that a new stratocumulus scheme increases marine stratocumulus cloud amount off the west coast of continents. Moreover, we have carefully tuned the whole cloud scheme such that the simulated climatological cloud radiative forcing (CRF) agree with the estimation from the Earth Radiation Budget Experiment (ERBE). Present-day climate simulation by the 20-km model was conducted for 10 years, forcing with observed climatological sea surface temperature (SST). The Earth Simulator was used for calculations. However, simulated low-level cloud was still underestimated compared with observations of International Satellite Cloud Climatology Project (ISCCP, 1983-2001), especially over the oceans. A new stratocumulus scheme was found to be not so effective as in the case of coarser resolution models. Increase of vertical resolution or horizontal resolution might suppress the westward extension of low-level inversion layer off the west coast of continents.