A new microreactor suitable for reaction crystallization was developed. In order to prevent particles from depositing on the channel wall, the reactants were injected into a stream of immiscible fluids, so that droplets consisting of the reactants were formed. This microreactor can be used to synthesize particles without fouling. The microreactor was applied to calcium carbonate precipitation and silver particle synthesis. Fouling did not occur, even when calcium carbonate precipitation was undertaken continuously for 9 h. In the silver particle synthesis, the particles obtained showed a narrower size distribution and a smaller degree of agglomeration. The fluid dynamics in the microreactor were also investigated using a computational fluid dynamics simulation. The mixing behavior in the droplets was affected by the flow conditions of the reactants.