Electrocatalytic activity of supported FePt alloy nanoparticles (NPs) with different compositions (Fe25Pt75, Fe30Pt70, Fe35Pt65) for the electro-oxidation of vitamin C is investigated. These spherical FePt NPs with nanocrystallite size of 7-9 nm are found to consist of a nanoalloy core with a more Pt-rich shell. The FePt alloy NPs are superior catalysts than Pt NPs for vitamin C electro-oxidation, with a linear concentration range of 0.01-1 mM, a high sensitivity of 4.347 mA cm(-2) mM(-1), and a low detection limit of 0.1 μM (S/N = 3). By effectively reducing the overpotential for the electro-oxidation, these alloy NPs are significantly more selective to the detection of vitamin C against other common interference species, including dopamine, citric acid, uric acid, glucose, and NaCl. Enhancement in sensor performance can be attributed to the increase in surface area due to reduction of nanocrystallite size and to modification in the Pt electronic structure as a result of nanoalloying. These are supported by the X-ray diffraction data and binding energy shifts as observed by X-ray photoelectron spectroscopy, respectively. Alloying therefore represents a powerful approach to introduce synergetic properties for new biosensor applications.