Abstract: This study examined the aircraft observations in stratocumulus and cumulus during the Routine AAF (Atmospheric Radiation MeasurementARM Aerial Facility) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign over the Southern Great Plains site near Lamont, Oklahoma, US, form 22 January to 30 June 2009. The probability density distributions of microphysical properties in the stratocumulus and cumulus were fitted and compared using Gamma, lognormal, and Weibull distributions. Futhermore, the impact factors to the relative dispersion of cloud droplet size distributions were discussed. Results showed that the three probability distribution functions fit the probability density distribution of microphysical properties in stratocumulus and cumulus clouds very well, and the coefficient of determination of most microphysical properties reaches more than 0.80. In general, the Gamma distribution has the best performance for the microphysical properties in the stratocumulus clouds, and the lognormal distribution has the best performance for the microphysical properties in cumulus clouds. Comparison of the distributions of microphysical properties between the stratocumulus and cumulus clouds showed that the cloud droplet number concentration in the cumulus clouds is higher and the average radius of cloud droplets is smaller, and the liquid water content, standard deviation and relative dispersion are comparable. In terms of the impact factors of relative dispersion, realtive dispersion in both stratocumulus and cumulus are negatively correlated with the cloud droplet number concentration and vertical velocity. The reasons are: when vertical velocity is large, supersaturation is expected to be high, which causes large number concentration and small relative dispersion, and small vertical velocity corresponds to strong entrainment and droplet evaporation, which further causes small number concentration and large relative dispersion.