Abstract: The separation performance of an innovative liquid - liquid hydrocyclone proposed by the authors was studied experimentally by using the Coulter counter technique. The number, size and concentration of oil droplets were measured, and the grade efficiency curves were determined for different oil contents, flow rates and overflow ratios. The results show that the separation performance of the hydrocyclone is improved with the increase of flow rate and overflow ratio. The performance of the proposed innovative hydrocyclone is obviously better than that of the F - type hydrocyclone proposed by Southampton University. The best separation performance is obtained at the flow rate of 10 m3·h -1 and overflow ratio of 3% - 5%, and the critical diameter of oil droplet to be separated is as small as 30 μm. Meanwhile, this diameter is about 60μm in the F - type hydrocyclone under the same test condition.