In the synthesis of metal oxide nanoparticles (NPs), methods to control their size and distribution have been desired. Precise and rapid control without the use of organic surfactants is a serious challenge. In this study, we report a strategy, dual-stage method using supercritical and subcritical water, for precise and rapid control of the size and distribution of NPs and applied it to the synthesis of CeO2 NPs. The synthesis was performed using three types of flow reactors, and the effects of temperature, residence time, and precursor concentration were examined. As a result, independent utilization of supercritical and subcritical water for separate reaction stages led to significant improvement in the size and distribution control relative to only utilization of supercritical water. The controlled size ranged from 6 to 22 nm, and the coefficient of variance was reduced from 0.32 to 0.14 within 5 s based on the uniform growth mechanism. Furthermore, growth simulations strongly support that the ideal growth, without the concurrence of nucleation, proceeded under specific precursor concentration.