The effects and influences of various operational parameters on the efficiency of mixer-settlers are thoroughly studied in this paper. One of the most important factors in designing a mixer-settler is to obtain an optimum operational extraction and overall efficiency. These two factors are dependent on various parameters such as impeller shape, size and speed of impeller, baffle size, residence time, volumetric flow rates and the ratio of heavy to light phases. In this paper, the effect of impeller speed and volumetric flowrates of two phases on efficiency are studied. Also, the effect of hold-up of each stage upon stage efficiency is studied. According to the above experiments and achieved results, it can be concluded that in general, an increase in impeller speed and solvent amount results in an increase in extraction efficiency. Also, when hold-up of the dispersed phase decreases, the stage efficiency usually increases up to a certain point.