Nano-catalysts, Au nano-particles on TiO2 (anatase), were studied by means of quantitative scanning transmission electron microcopy (Q-STEM) and high-resolution transmission electron microcopy (HRTEM). All the Au nano-catalysts were produced from an Au13 precursor, Au13[PPh3]4[S(CH2)11CH3]4, TiO2 supports, with three treatments: (1) thermal heating in the air at 400°C for 2 hours, (2) exposure to ozone (O3) at room temperature, and (3) exposure to atomic oxygen (AO, or O) with a AO dose of 7.3 ×1018 atom/cm2 at room temperature. Both reactive oxygen species O3 and AO produced significantly small sizes of Au particles as compared to those from the heating treatment in the air (2.7 ± 0.6 nm, 324 b 264 atoms). Ozone produced the smallest (1.2 ± 0.5 nm, 40 ± 49 atoms), whereas AO produced smaller (2.1 ± 0.7 nm, 72 ± 98 atoms), with a broad size distribution and a variety of shapes. HRTEM studies on the AO treated Au/TiO2 samples found that there could exist relationship between the particles size and their shapes which also affected by the interaction with TiO2 supports. The support effect of TiO2 to the shapes of Au particles was also studied.