Carbon nanotubes (CNT) were used as a catalyst support where catalytically active Pd and Pt metal particles decorated the outside of the external CNT walls. In this study, Pd and Pt nanoparticles supported on HNO3-treated CNT were prepared by microwave-assisted heating of the polyol process using PdCl2 and H2PtCl6.6H2O precursors, respectively, and were then characterized by SEM, TEM, and Raman. Raman spectroscopy showed that the acid treated CNT had a higher intensity ratio of ID/IG compared to that of nontreated CNT, indicating the formation of defects or functional groups on CNT after chemical oxidation. Microwave irradiation for total two minutes resulted in the formation of Pd and Pt nanoparticles on the acid treated CNT. The sizes of Pd and Pt nanoparticles were found to be less than 10 nm and 3 nm, respectively. Furthermore, the SnO2 films doped with CNT decorated by Pd and Pt nanoparticles were prepared, and then the NO2 gas response of these sensor films was evaluated under 1~5 ppm NO2 concentration at 200oC. It was found that the sensing property of the SnO2 film sensor on NO2 gas was greatly improved by the addition of CNT-supported Pd and Pt nanoparticles.