The petroleum refining industry produces a large amount of spent catalysts which contain sulfur and some valuable metals. During the recovery of these valuable metals, sulfur should be processed and removed from the surface of the spent catalyst. Several chemical reagents including NaOH, Na2CO3, H2O2, CS2, and decanol were employed to investigate the removal of sulfur from the spent catalysts. NaOH and ultrasound-assisted oxidative treatment with H2O2 showed better performance than the other reagents for the removal of sulfur from spent catalyst. Sulfur and valuable metals dissolved together to the solution by using ultrasound-assisted oxidative treatment with H2O2, while the covered sulfur was removed from the spent catalyst without affecting the dissolution of valuable metals. The removal of sulfur as well as the leaching of valuable metals was discussed on the basis of oxidation and reduction reaction. The optimum conditions for sulfur removal were chosen as follows: temperature, 60°C; pulp density, 20 g/dm3; reaction time, 1.5 h; stirring speed, 800 rpm; and NaOH concentration, 0.5 M. The obtained results can be applied to develop an environment-friendly process to treat sulfur from the spent catalysts.