A novel reprocessing process based on sulfide chemistry has been proposed for the recovery of nuclear materials from spent fuel. To apply the sulfide process in the thorium fuel cycle, the sulfurization behavior of thorium dioxide (ThO2) with CS2 was examined as a basic study. When thorium dioxide powder reacted with a mixture gas of Ar and CS2 at different temperatures in a quartz reaction tube, the phases after the reaction were identified by the X-ray diffraction method. At temperatures lower than 673 K, ThO2 was found to be stable because the lattice parameter of the ThO2 phases does not change with increasing sulfurization temperatures. However, the weight gradually increased, suggesting that a small amount of sulfur was trapped in the lattice from the sulfurization. The formation of the intermediate phase ThOS was observed at 973 K. At 1073 K, both the ThOS and ThS2 phases were observed. Finally, a single phase of ThS2 was obtained at temperatures higher than 1173 K. The obtained results were compared with the thermodynamic consideration by using the potential diagram of the Th-S2-O2 system and the experimental results were in good agreement with the thermodynamic considerations.