Introduction Having an efficient and reliable system for oil-water separation is of fundamental importance especially for the offshore oil and gas industry. Due to platform motion, space, weight and operational limitations offshore, the use of common methods (gravity vessels) for oil/water separation is ineffective. On the other hand, oil production is often accompanied by large quantities of water being discharged into the sea on offshore platforms. The amount of oil in the water is limited by environmental standards. Therefore, the need to have a compact separator with high efficiency under variable operating conditions attracts researchers' interest towards hydrocyclones. The special features of hydrocyclones such as simple design, ease of installation and operation, no moving parts, and low manufacturing and maintenance costs make hydrocyclones an economical and effective system for treating produced water [1-2] . The separation process that occurs in oil-removing hydrocyclones is very different from sand-removal hydrocyclones []. The difference in density between liquid-liquid is smaller than in solid-liquid mixtures. Therefore the separation of liquid from liquid is more difficult than that of solid from liquid and requires higher rotation speeds. The drop of liquid could not undergo shear velocity compared to solid particles. If the shear rate increases to the critical level, larger droplets split into smaller ones. The process of separating small liquid droplets is more difficult than large ones. On the other hand, if two droplets close enough together they might merge together. Due to the difference in flow splitting of sand trap and oil trap hydrocyclones, the flow characteristic of the continuous phase is different in these two types. Centrifugal forces cause solid particles to migrate towards the wall region in sand-draining hydrocyclones. Thus the near-wall region is important in desilting hydrocyclones, but oil droplets migrate to the center of the hydrocyclones in desilting hydrocyclones and consideration tends towards central flow characteristics. The first idea of ​​using common hydrocyclones for oil-water separation was suggested by Simkin and Olney [6] and Sheng et al. [7] but fundamental studies on oil-removing hydrocyclones began in 1980 by Colman [8] and Thew. Several experimental investigations on oil-removing hydrocyclones have been conducted by Colman et al. [9] and Colman and Thew [10-12]. The results showed that the separation efficiency of hydrocyclones is independent of flow splitting between 0.5 and 10%. So the overflow diameter should be designed according to the working conditions. Furthermore, for a constant size distribution of the inlet droplets, the outlet size distribution is independent of the flow subdivision.