Waste water resulted from polymer flooding oil recovery generally has a bad impact on the subsequent process of enhanced oil recovery. Separating residual oil from oil/water (O/W) emulsion with suitable kinds of demulsifier is one strategy generally adopted by oil companies. Because of the existence of large amounts of ultrafine oil droplets with the average diameter less than 2 pm, the emulsions can be extremely difficult to break up. To solve this problem, an amine-based dendrimer demulsifier PAMAM (polyamidoamine) was synthesized in this study, and the efficiency of the demulsifier in dealing with O/W emulsions with ultrafine oil droplets was investigated. Because of its strong interfacial activity and relatively good solubility in water, the dendrimer-based demulsifier can easily attach to emulsified oil droplets in a stable diesel-in-water emulsion. The influences of temperature, settling time, and concentration of the demulsifier used on the efficiency of the demulsifier were investigated in detail. The optimal operating condition under which more than 90% oil was removed from the original emulsion by the demulsifier was found. In contrast, less than 2% oil was removed from the emulsion without applying the demulsifier under the same conditions. Micrographs showed that the PAMAM demulsifier could lead to the breakup of diesel-in-water emulsions with ultrafine oil droplets by flocculation and coalescence. The surface tension and interfacial tension at the diesel-water interface were also measured to give a basic understanding of the demulsification mechanism. Though not perfect in dealing with emulsions with the average oil droplets less than 2 mu m due to the relatively high demulsifier dosage, its relatively simple synthetic procedure and mild operating conditions showed a great promise in industrial applications with unique advantages over traditional physical methods.