Dendrimer encapsulated Pt nanoparticles were prepared by using hydroxyl terminated generation four (G4OH) PAMAM dendrimers (DEN) as the templating agents. The encapsulated Pt nanoparticles were dispersed on gamma-Al2O3 at room temperature by impregnation. Pt/Al2O3 (DEN) catalysts were then subjected to thermal treatments in oxidizing and reducing atmospheres at different temperatures. These catalysts were characterized by Transmission Electron microscopy (TEM) and In situ Fourier-Transform Infrared (FTIR) spectroscopy. The TEM analysis of the as synthesized catalysts revealed that the Pt nanoparticles were found to be 2-4 nm in size. It is observed that the Pt particle size in 0.5% Pt/Al2O3 (DEN) catalyst increased upon thermal decomposition of the dendrimer. The in situ FTIR results suggested that the presence of oxygen and the Pt nanoparticles in the Pt-dendrimer nanocomposite accelerate the dendrimer decomposition at low temperatures. All the catalysts were tested for the reduction of NOx with CH4 in the temperature range of 250-500 degrees C. NOx reduction efficiency of Pt/Al2O3 (DEN) catalysts were compared with the Pt/Al2O3 (CON; conventional) catalyst. The conversion of NOx was started from the low temperatures over Pt/Al2O3 (DEN) catalysts. The high selectivity of NOx to N-2 of 74% was obtained over 0.5% Pt/Al2O3 (DEN) catalyst at low temperatures around 350 degrees C.