Highly dispersed ruthenium (Ru) nanoparticles supported on activated carbon (AC) were controllably synthesized by supercritical fluid deposition (SFD). The Ru nanoparticles prepared by SFD presented a smaller mean particle size than particles prepared by the ethanol impregnation method. The effects of temperature, pressure, and Ru loading on catalyst preparation were systematically investigated. As any one of above parameters increased, Ru mean particle sizes always decreased first and then increased. The smallest mean particle size (1.6 nm) was obtained at Ru loading of 2%, 45 degrees C, and 10 MPa. On the scale of Ru mean particle size from 1.6 to 3.9 nm, the catalysts with smaller Ru mean particle size presented higher catalytic activity. The interactions between the Ru precursor and AC were characterized by TG-MS, TPR, and FTIR spectroscopy. The results obtained have revealed the reduction of the carbonyl (C=O) in Ru precursors after depositing on AC with the aid of SCCO2.