We studied extremely fast kinetics of an outer-sphere heterogeneous electron transfer (ET) reaction at a single Pt nanoparticle (NP) using the newly adapted Kotecky-Levich (K-L) method. In this work, an electrode was prepared by nucleating and growing a single Pt NP on a tunneling ultramicroelectrode (TUME) that produces 1-40 nm or greater dimensions. Such a small-size electrode greatly enhances the mass transfer rate, thus enabling us to reliably determine ET kinetic parameters for fast ET reactions. Based on the recently demonstrated K-L model for a general UME, ET kinetic information could be measured by constructing a plot of 1/current density vs 1/mass transfer rate from the series of steady-state voltammograms obtained using Pt NP-deposited TUMEs. For this K-L plot, we altered the mass transfer rates by varying the electrode size, i.e., the Pt NP size in this work. The determined standard rate constant, k(0), of heterogeneous reduction reaction for Ru(NH3)(6)(3+) was unprecedentedly high, at 36 +/- 4 cm/s, confirmed by theoretical simulation. Extended applications to various electrocatalytic reactions with different types of electrodeposited metal NPs will show the versatility of our approach. Particularly, this novel fabrication of a nanometer-sized electrode and its application to fast ET kinetic study with simple instrumentation should be useful in studies of particle size and structure effects on given catalytic reactions.