Abstract: Extraction factor is one of the most important parameters in optimum design of countercurrent extraction technology.The precise equation for the optimized extractor factor has been deduced in this paper.The calculation error of the optimized extractor by the precise equation is lower than the error by the simple equation.If the separation factor is below three,the optimized extractor can be calculated by both of the simple equation and the precise equation.If the separation factor is over three,the precise equation is more suitable for calculating the optimized extraction factor.