Carbon nanotube (CNT)-supported Pt nanoparticle catalysts have been synthesized in supercritical carbon dioxide (scCO(2)) using platinum(II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that the platinum particles' size is in the range of 5-10 nm. XPS analysis indicates the presence of zero-valence platinum. The Pt-CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt-CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt-CNT from scCO(2) provides a strong electrocatalytic activity for oxygen reduction reaction. For the methanol oxidation reaction, a high ratio of forward anodic peak current to reverse anodic peak current was observed at room temperature, which implies good oxidation of methanol to carbon dioxide on the Pt-CNT electrode. This work demonstrates that Pt-CNT nanocomposites synthesized in supercritical carbon dioxide are effective electrocatalysts for low-temperature fuel cells.