The effect of Al2O3 as non-soluble sintering aid of coarse-grained and fine-grained ThO2 has been explored under oxidizing and reducing atmospheres. It was found that Al2O3 increases the sinterability of coarse-grained ThO2 powder substantially under both oxidizing and reducing conditions. Sintering of fine-grained ThO2 powder based pellets resulted in slightly lower densities upon doping with Al2O3 compared to the undoped system. Precise X-ray diffraction analysis of the unit cell parameter and microprobe investigations ascertained that Al is not incorporated in the ThO2 matrix under any of the experimental conditions. X-ray diffraction measurements on the sintered ThO2 gave 559.74 +/- 0.01 pm as the cubic unit cell parameter. It was further observed that the sintering atmosphere has a limited, but measurable effect in the late sintering stage of pure ThO2.