The drop size in a mixing vessel is dependent on the micro- and macro-scale turbulent motions and flow patterns in the vessel. Accordingly, introduction of aeration can change the drop size distribution because the aeration changes the flow patterns and the turbulent motions in the mixing vessel. Experimental results in this study show that both small- and large-size drops in distribution increase with aeration while the fraction of the intermediate size drops decreases. It is also observed that the drop size distributions are not influenced by the amount of aeration rate though introduction of aeration gives remarkable change. The volumetric drop size distribution in the mixing vessel with aeration is represented by a combination of three normal distribution curves though the number distribution curve is represented by a combination of two log-normal distribution curves. The effect of energy dissipation rate by agitation and that of volume fraction of dispersed phase on the Sauter mean drop size shows that coalescence phenomena dominate the dispersion of drops in the mixing vessel with aeration.