The effect of critical pressure was investigated in order to control the size and size distribution of droplets in an emulsion made by microfiltration. For this study, O/W emulsions composed of kerosene and SDS solution and Nuclepore membranes having nominal pore diameters of 0.6, 0.8, 1.0 and 3.0 mum were used. In the case of emulsion droplets, the sizes and shapes can be changed by pressure, unlike rigid particles. Droplets having a size larger than the membrane pores can permeate-or be destroyed-through the membrane above the critical pressure. In the present study, a new model of critical pressure considering the influence of droplet destruction was proposed. The validity of this model was investigated by comparisons between measured and calculated critical pressure values and between the size distributions on the feed and permeate sides. It was found that in some regions droplets larger than the pore passed through pores without destruction, whereas in other regions they were divided into several parts.