In hydrothermal synthesis of nano-size particles, in which an aqueous metal salt solution at a normal temperature and supercritical water are directly mixed, the mixing process of the two fluids is extremely crucial for obtaining a highly supersaturated crystallization field and growth process control of fine metal oxides. This paper reports on a newly developed swirling micro mixer as a replacement of a conventional mixing tee-union for improved mixing of the two fluids. The developed micro mixer was featured with segmented introducing of supercritical water, swirling and convergent nozzle flow, and was examined by using CFD simulation. In a practical experiment of boehmite (AIOOH) fine particle synthesis, the developed micro mixer demonstrated average particle sizing down from 100-200 nm to 60 nm (by TEM and DLS data) in comparison with the reference experiment data that used a conventional mixing tee. In addition, a reduced secondary aggregation and high dispersibility were also demonstrated. The experimental results show the swirling micro mixer works well for the synthesis of desired particles and the reduced possibility of channel blockage by particle aggregated solids.