We demonstrate highly efficient electocatalytic activities of single crystalline RuO(2) nanorods grown on carbon fiber (CF), i.e., RuO(2) nanorod-CF hybrid microelectrode, prepared by a simple thermal annealing process from the Ru(OH)(3) precursor at 300 °C. The general electrochemical activity of a RuO(2) nanorod-CF microelectrode represents faster electron transfer for the [Fe(CN)(6)](3-/4-) couple than that of the bare CF microelectrode which are confirmed from the cyclic voltammetry (CV) measurement. Also, the amperometric response for the H(2)O(2) oxidation is remarkably facilitated at the RuO(2) nanorod-CF microelectrode by not only the enlarged surface area but the high electrocatalytic activity of the RuO(2) nanorod material itself. Furthermore, a single microelectrode of RuO(2) nanorod-CF exhibits the superior tolerance to Cl(-) ion poisoning unlike Pt-based electrocatalysts, indicating the promising sensor candidate in physiological conditions.