The mixing process of metal salt solutions and supercritical water is essential to supercritical water hydrothermal synthesis (SWHS) to produce nanoparticles. A computational fluid dynamics (CFD) model was developed for predicting mixing efficiency and crystallization kinetics in SWHS mixers. The mixing efficiency was calculated from the micromixing model, and the kinetics model of crystallization was built from the population balance equation. The effects of the fluid dynamics of mixing (Reynolds number and mixer diameter) on the mixing efficiency and crystallization kinetics (nucleation rate and growth rate) were investigated. The results showed that faster mixing can lead to a higher crystallization rate and the production of smaller particles with a narrower particle size distribution.