Abstract: The relationship between mean inlet drop size and separation efficiency of reducing section,tapered section,parallel section and the whole hydrocyclone is discussed in this paper.The effect of operation parameters such as feed flowrate, split ratio on mean oil drop size in the wall of each part in hydrocyclone is also investigated.The result suggests that separation efficiency of the reducing section,tapered section and the whole hydrocyclone increased with increasing inlet droplet size,whereas the separation efficiency of parallel section does not vary very much.When inlet flowrate is high enough,mean droplet sizes in the wall of hydrocyclone decreases with increasing feed flowrate.The split ratio has little influence on mean droplet sizes on the wall of hydrocyclone,and mean oil droplet diameters decreases along the axial direction under same operation condition.