To avoid a poor mixing rate and local inhomogeneities in batch reactor systems, and to shorten the length of microchannels in microreactor systems, a new combined micromixer/microreactor/batch reactor system was used for the synthesis of colloidal silica particles. The silica particles with different sizes (from 20 nm to 2 μm) and size distributions (which were characterized by PDI from 0.01 to 0.40) were controllably synthesized by varying the concentration of reactants and the operating parameters in this system. The long microchannel with small diameter demonstrated a good mixing efficiency, and which produced small and uniform silica particles. In addition, the introduction of inert gas into the system intensified the mixing, and the silica particles with decreased size and narrow distribution were obtained. It was clearly demonstrated that the high mixing efficiency in the microchannel led to small and uniform silica particles. Furthermore, a theoretical foundation for the synthesis of nanoparticles in microreactors was established after illustrating the relationship of mass transfer and reaction in the system.