Abstract: To improve the dispersion stability of carbon blacks in water, polymer grafting on the surface of the blacks is commonly used. A free radical polymerization method was first used to graft a co-polymer of acrylic acid and a polymer containing sulfonic groups onto the surface of a furnace black under ultrasonic irradiation. The optimized conditions (pH, content of initiator and acrylic acid, reaction time) for high grafting ratios have been obtained. With the aid of XPS, FTIR and SEM analysis, the difference in surface functional groups and morphologies of the carbon blacks before and after grafting was investigated. It was found that ultrasonic waves significantly reduced the grafting time of acrylic acid. With the ultrasonic irradiation, the force acting between the as-grafted carbon blacks and water and the static repulsion among the carbon black particles were increased to a large degree. Together with the steric hindrance caused by the polymer molecules, the dispersion stability of carbon black in water was obviously improved.