Sulfate radical-based advanced oxidation processes have had considerable attention due to the highly oxidizing function of sulfate radicals (SO4-·) resulting in acceleration of organic pollutants degradation in aqueous environments. A Co-Ni mixed oxide nanocatalyst, which was prepared by the sol-gel method, was employed to activate peroxymonosulfate (PMS, HSO5-) to produce SO4-· with Acid Orange 7 (AO7) selected as a radical probe. The catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). The characterization results indicated that the ingredient of the catalyst had been changed and the amount of surface hydroxyl increased significantly with the addition of Ni. Therefore, it proved that Co-NiOx catalyst was more effective than CoOx to activate PMS. Moreover, ultrasound (US) can increase the degradation rate of AO7 and US/Co-NiOx/PMS system. This study also focused on some synthesis parameters and the system reached the maximum efficiency under the condition when [PMS] = 0.4 mM, [catalyst] = 0.28 g/L, Pus = 200 W. The AO7 removal in these systems follows first order kinetics. Last but not least, quenching studies was conducted which indicated that the amount of hydroxyl radicals (·OH) increases with the increase of initial pH and SO4-· was the primary reactive oxidant for AO7 degradation.