Supercritical water impregnation is a novel method, which utilizes high diffusivity of the fluid with hydrothermal synthesis allowing nano-particles to deposit on porous materials in well-dispersed condition. In this work, silver nano-particles were deposited on the surface of alpha-alumina supports using metal acetate solution. TEM-EDS analyses clearly identified the silver particle of two kinds, those deposited on alpha-alumina surface and those gathering in bulk phase. Their particle size distribution showed differences in propensity, implying that particle formation were via different mechanism. The difference in particle deposition mechanism was also confirmed from the results of other operational factors investigated. Furthermore, the mechanism of particle deposition to alumina support surface have been proposed based on the experimental results and reported observation of nucleation and particle growth steps during the hydrothermal synthesis. In addition, the mechanism of silver particle formation has been discussed in detail.