We demonstrated an efficient method for size-controlled nanoparticles of the open framework coordination polymer potassium copper hexacyanoferrate (KCuHCF) using only aqueous solutions of the raw materials and a Y-type micro-mixer. Despite the high viscosity of the synthesized NP slurry, the micro-mixer provides continuous synthesis without clogging for a few hours with a high flow rate of 100 mL min⁻¹, i.e. a linear velocity of 94 m s⁻¹. The crystallite size, evaluated by the Scherrer equation using X-ray diffraction measurements, can be controlled by changing the flow rate. With the highest flow rate of 100 mL min⁻¹, the smallest NPs with a size of ∼11 nm were obtained, less than half of the NP size obtained using the batch method. By downsizing the nanoparticles (NPs) using the micro-mixer synthesis, the Cs adsorption performance of potassium copper hexacyanoferrate (KCuHCF) was drastically improved. The KCuHCF with the smallest primary particles showed the fastest Cs adsorption: 1.4 times in the saturated capacity, 3.9 times in the distribution coefficient, and 7.7 times in the rate constant for the pseudo-second order adsorption theory, compared with the batch-synthesized sample.