The effects of the number of discs and the operating conditions on the separation performance of an oil-water separator with conical discs were examined. The estimated value of the partial separation efficiency obtained under the assumption of a plug flow model was compared with the measured value. The difference between the two values was decrease with the number of discs and increase with the inlet flow rate of the separator. The estimated overall removal efficiency of oil, which was calculated on the basis of the estimated partial separation efficiency and the measured size distribution oil droplets of was up to 25% higher than the measured value.