The optical properties of heavily Al-doped single-crystal Si3N4 nanobelts are investigated by measuring their absorption, photoluminescence, and photoluminescence excitation spectra. The results suggest that the doped Si3N4 exhibit two absorptions at 2.5 and 4.2 eV, instead of absorption at 5.0 eV in the pure Si3N4. The doped nanobelts show light emissions in a range of 1.4 to 3.6 eV, which is red-shifted as compared with that of pure Si3N4 nanobelts. These results are ascribed to the unique doping mechanism of Al, which generates two types of defects.