A unit molar weight mass difference diffusion equation was derived and solved by the radial averaging approximation method for binary component isotopes. There was no limit to the range of concentration value. The expression of gamma(0) was given for arbitrary concentrations. In addition, the analytical solution of a purely axial flow in a gas centrifuge was obtained by a linearized motion equation for a centrifuge. The solution was not under the condition that the speed parameter A(2) >> 1. Calculation examples show the distributions of different magnitudes of A(2). The parameters that influence the value of gamma(0), such as the feed flow rate, F, the product, rho D, the speed parameter, A(2) and the wall pressure, p(w), are discussed.