A novel motionless mixer named the Ramond Supermixer(R) (RSM(R)) was employed to produce O/W emulsions composed of micrometer and submicrometer-size droplets. Liquid paraffin as dispersed phase, aqueous sucrose solution as continuous phase, and anionic sodium dodecyl sulfate as emulsifying agent were used as the model emulsification system. Pressure drop, droplet size distribution, Sauter mean diameter (d(32)), and geometric standard deviation of the droplet size distribution (sigma(g)) were investigated under the various combinations of process variables; superficial liquid velocity, number of mixing units, number of passages through RSM(R), dispersed phase viscosity (eta(d)), continuous phase viscosity (eta(c)), and dispersed phase volume fraction. Different modes of droplet size variations with process variables were obtained, with respect to micrometer- and submicrometer-size ranges, and theoretical explanations are forwarded. For the micrometer- size range, maximum droplet diameter (d(max)) was proportional to d(32). For the submicrometer-size range, d(max) varied with d(32) in the range of 1.53-2.19-fold, and a correlation is proposed with K (=eta(d)/eta(c)); d(32) and sigma(g) were well correlated with the process variables. Furthermore, a semi-empirical mechanistic model was developed for the formation of droplets obtained under inertial sub-range to interpret the effect of process variables.