The effect of a tetravalent dopant, Th4+, on the oxidation of UO2 was investigated using a thermogravimetry and X-ray diffraction analysis. Th-doped UO2’s with various dopant contents were prepared and their oxidation kinetic curves were obtained from the weight gains during air-oxidation. For the first oxidation step from (U1-yThy)O2 to (U1-yThy)4O9, the oxidation kinetic curves showed the same gradient regardless of the Th content. The inhibition of the oxidation reaction occurred dominantly in the second step, from (U1-yThy)4O9 to (U1-yThy)3O8. At the plateau of the second stage, the calculated O/M values and the X-ray diffraction patterns revealed that the O/M ratio was decreased with an increase of Th content. The relationship between the mean formal charge and the composition showed that the oxidation to (U1-yThy)3O8 proceeds within a certain limit. The lattice parameter of the initial material seems to affect the oxidation rate of the first step. And the oxidation reaction stopped when the average formal charge of the U atoms reached a value of 5.3.