Abstract: Coaxial co-flow capillary microreactors were designed and used to prepare fluorescent polymer dots (Pdots) using poly(9,9-dioctylfluorene-co-benzothiadiazole) (PFBT) as a polymer, tetrahydrofuran (THF) as a solvent, deionized water as an antisolvent, polystyrene - maleic anhydride copolymer (PSMA) as a stabilizer by a nanoprecipitation method. The size distributions of Pdots were measured by a dynamic light scattering (DLS) technique. The results showed that the sizes of polymer nanoparticles decreased as the microchannel diameters decreased in the range of 50 μm ~ 500 μm. For 1 μg/mL～50 μg/mL PFBT solutions, the sizes of polymer nanoparticles decreased with decreasing the mass concentration of the polymer. The diameters of polymer nanoparticles were reduced low to 14 nm using a 5 μg/mL PFBT solution at a flow ratio of deionized water to polymer solution of 40:1 in a microchannel with a diameter of 50 μm. The sizes and size distributions of nanoparticles were controlled accurately using the microfluidic technique presented here.