A dual-pipe-type microreactor was applied to the formation of nanoparticles at a very rapid reaction rate. As a representative example, zirconia particles were produced by supplying zirconium tetrabutoxide solutes and alcohol/water mixtures. The effects of the solvent and alcohol types and the residence time on the particle properties were examined. Zirconia particles were produced and grown in an inner fluid, and no precipitation of particles was observed at the wall in the outer fluid. When ethanol was used as a solvent for diluting water, the zirconia particles had a sharp narrow size distribution that could not be attained by means of a conventional batch method. The mean particle size was successfully controlled in the range of 4-600nm merely by changing the mean residence time and the concentrations of zirconium tetrabutoxide and water in the presence of a dispersant, polyethyleneimine.