Abstract: The model of the fractional counter current extraction (FCCE) is modified based on the study of the extraction mechanism. In this model 2n equations for the equilibrium acidity (H(I)=H₀-nX:(I)) and extractant concentration (Z(I)=Z₀-nY:(I)) are subs-tituted into the FCCE model proposed by the authors earlier (1) and the number of the equations can be reduced from 2n (m+1) to 2nm. An improvement in the restriction and iteration for the solution is taken, making the physical meaning of the solution more accurate and thus the stability of converge-nce. A mathmatical simulation for HEH(EHP)-kerosene-HNO₃-(Sm-Gd)(NO₃)₃FCCE sys-tem is given and the calculated values are more fit with the experimental data than in earlier work. The influences of the extraction conditions on the separation are also dis-cussed. It is shown that this model can be applied to computer aided design.