Studies on emulsification, have hitter to dealt mainly with droplets of micrometer size. Very few reports concern emulsification into droplets of sub-micrometer size, though recently a few studies have been reported on emulsification by means of high-pressure homogenizers which can prepare such emulsions. In the present work, oil in water (o/w) emulsions composed of droplets of sub-micrometer size were produced by means of a high pressure wet-type jet mill, and the relation of mean droplet size to operating variables was discussed. Furthermore, the time dependency of droplet size was discussed on the basis of a kinetic model. The drop size distribution (volume basis) showed a log-normal distribution. As the number of passages increased, the drop size distribution became bimodal, due to the increase of extremely fine droplets. When the number of passages was unity, Sauter mean diameter (d(32)) increased with increasing dispersion phase fraction, but the geometric standard deviation remained unchanged. For the present experimental conditions, the maximum droplet diameter (d(max)) based on droplet number was expressed in the form of d(max)=1.76d(32). At low surfactant concentrations and low dispersion phase fractions, d(32) was proportional to the applied pressure raised to the power of ca.-0.6. The drop size distribution exhibited the time-dependence because of the coalescence of droplets. The process of coalescence was judged to be described by a second-order process consisting of a rapid coalescence process and a slow coalescence process. The slow coalescence process was realized under the conditions of high processing pressure, high surfactant concentration, low dispersed phase fraction and large number of passages. It was also found that the coalescence rate constant increased with increasing dispersed phase fraction and decreased with increasing surfactant concentration.