Abstract: The coalescence of water drops of Chinese Luning Pipeline crude oil in AC or DC electrostatic field is studied. The effects of voltage gradient and residence time on drop diameter and water content in oil are investigated. The results show that the mean diameter and the maximum mean diameter of droplets increase slowly and then reach a constant value with increasing residence time under low voltage, but increase to a maximum rapidly and then decrease under high voltage. The higher the voltage gradient, the more rapid the rate of increasing and decreasing. There is an optimal residence time for the efficiency of dewatering, depending on voltage gradient. For insulated electrodes, the efficiency of coalescence in AC electrostatic field is better than that in DC electrostatic field.