A variety of loadings of Cobalt oxide (Co₃O₄) was successfully supported on the reduced graphene (Co₃O₄/rGO) as the catalyst to activate peroxymonosulfate (PMS) to generate sulfate radicals (SO₄⁻•) for degrading Orange II in water. The crystal structure of Co₃O₄/rGO with different loadings was characterized by X-ray diffraction (XRD), and their catalytic activity was compared in the same conditions. The result showed that the catalyst has an optimum Co₃O₄ loading. Using the 70.7% loading of Co₃O₄ in Co₃O₄/rGO as the catalyst, 100% decomposition could be achieved within 4 min with 1 mM Orange II, 0.05g/L catalyst, and 10 mM PMS.