High-density polyethylene/fullerene (HDPE/C60) nanocomposites with the C60 loading that varied from 0.5 to 5.0% by weight were prepared via melt compounding. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results showed that the presence of C60 could remarkably enhance the thermal properties of HDPE. A very low C60 loading (0.5 wt%) increased the onset degradation temperature from 389°C to 459°C and decreased the heat release from 3176 J/g to 1490 J/g. The larger the loading level of C60, the better the thermal stability of HDPE/C60 nanocomposites. Rheological investigation results showed that the free radical trapping effect of C60 was responsible for the improved thermal stability of HDPE.