Understanding the relationship between the structure and the reactivity of catalytic metal nanoparticles (NPs) is important to achieve higher efficiencies in electro-catalytic devices. A big challenge remains, however, in studying these relations at the individual NP level. To address this challenge, we developed an approach using nanometer-scale scanning electrochemical microscopy (SECM) for the study of the geometric property and catalytic activity of individual Pt NPs in the hydrogen oxidation reaction (HOR). Herein, Pt NPs with a few tens to a hundred nm radius were directly electrodeposited on a highly oriented pyrolitic graphite (HOPG) surface via nucleation and growth without the necessity of capping agents or anchoring molecules. A well-defined nanometer-sized tip comparable to the dimensions of the NPs and a stable nanogap between the tip and NPs enabled us to achieve lateral and vertical spatial resolutions at a nanometer-scale and study fast electron-transfer kinetics. Specifically, the use of two different types of redox mediators: (1) outer-sphere mediator and (2) inner-sphere mediators could differentiate between the topography and the catalytic activity of individual Pt NPs and measure a large effective rate constant of HOR, k(eff)(0) of >= 2 cm/s as a lower limit at each Pt NP. Consequently, the size, shape, spatial orientation and the catalytic activity of Pt NPs could be determined at an individual level in nanoscale SECM where imaging accompanied by theoretical modeling and analysis. This approach can be easily extended to quantitatively probe the effects of the surface property, such as capping agent effects on the catalytic activity of a variety of metal NPs for the design and assessment of NP catalysts.