This paper presents a microfluidic method for precise control of the size and polydispersity of surfactantDNA nanoparticles. A mixture of surfactant and DNA dispersed in 35% ethanol is focused between two streams of pure water in a microfluidic channel. As a result, a rapid change of solvent quality takes place in the central stream, and the surfactant-bound DNA molecules undergo a fast coilglobule transition. By adjusting the concentrations of DNA and surfactant, fine-tuning of the nanoparticle size, down to a hydrodynamic diameter of 70 nm with a polydispersity index below 0.2, can be achieved with a good reproducibility.