Mixing condition and size distribution of atmospheric aerosols were investigated for determination of suitable optical aerosol model. If the geometric number mean radius and the standard deviation were 0.055-0.079μm and 1.97-2.29 in internal mixing condition respectively, calculated values of the ratio of diffuse solar radiation to direct solar radiation correlated well with those observed. Whereas the geometric number mean radius of 0.035 μm and standard deviation of 2.10, which have been recommended, gave less correlation. The difference of calculated single scattering albedo between external and internal mixture was 0.03, which has the possibility of the difference of 0.15°C of the global mean surface temperature.