The oil/water separation in a liquid-liquid cylindrical cyclone is experimentally studied in this article. The effects of the flow split-ratio and the flow rate on the oil/water separation performance are determined. From the experimental results, it is shown that with the increase of the flow split-ratio, the oil/water separation efficiency is enhanced at first, and an optimal flow split-ratio exists, beyond that optimal split-ratio, the watercut in the underflow keeps constant, while the oil content in the overflow begins to decrease. The process of the oil core structure formation and the phase distribution in the cyclone are determined by numerical simulations. Furthermore, the dependence of the separation efficiency on the Reynolds number and the flow split-ratio is investigated based on a dimensional analysis. A comparison between the predicted values and the experimental data shows a good agreement.