ThO2:Eu3+ nanoparticles were synthesized at 300 degrees C by combustion route using urea as a fuel and characterized by thermogravimetric/differential thermal analysis, X-ray diffraction, transmission electron microscopy, and photoluminescence techniques. To investigate the effect of annealing temperature, as synthesized powder were heated further at 500 degrees C, 700 degrees C, and 900 degrees C. It was observed that extent of asymmetry around Eu3+ at 700 degrees C/900 degrees C is very high as compared to as-prepared or 500 degrees C annealed sample. Based on the time resolved emission spectroscopic investigations, it was inferred that two different types of Eu3+ ions were present in the ThO2 nanoparticles. In ThO2 structure, Eu3+ ions occupy two sites; cubic (O-h) and noncubic (