A novel complexing agent, an oil-soluble polyaminocarboxylic acid alkylderivative, was utilized to enhance the mutual selectivity of rare earth metals in liquid-liquid extraction. This agent has a surfactant-like interfacial activity and strongly adsorbs at an oil-water interface. A novel extraction process combined with electrostatic coalescence to promote phase separation was proposed. Extraction rates of three rare earth metals (Y, Er and Yb) in the presence of the complexing agent with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as the extractant were measured by using this novel extraction method. The selectivity of Y over Er was significantly enhanced by the addition of a small amount of the complexing agent because the extraction rate of Y selectively decreased. The addition of the complexing agent had no influence on the extraction equilibrium. The extraction rate was analyzed and it was proved that the extraction rate is greatly influenced by the interaction between the metal ions and the complexing agent at the oil-water interface. Efficient extraction and separation of rare earth metals were performed with the newly developed extraction method.