A Monte Carlo simulation was used to estimate the particle size distribution in the synthesis of nanoparticles using microemulsions. When two microemulsions each containing reactants were mixed together, continuous collisions of the droplets lead to a particle-forming reaction through a repeated coalescence and breakup process. The particle formation in this process typically was limited by the droplet collision, because the mixing and reaction of the reactants were fast once two droplets coalesced. The present simulation indicates that a significant number of fine particles is generated making a broad hump in the particle size distribution on the small particle side. However, the volume fraction of the fine particles is insignificant, and the particle size distribution may be considered to be a narrow one if the fine particles are neglected. This prediction appears to be consistent with experimental observations, although only a limited number of experimental data is available.