The expressions of diffusion equation for multicomponent isotope separation by gas centrifuge are derived by utilizing the simplified diffusion transport vector. A method of radial averaging which was restricted to a binary mixture is extended to multicomponent isotope mixtures by using an iterative scheme. A numerical analysis of tetradic UF6 or SF6 gas isotope separation by centrifuge is discussed when a special model of velocity distribution is given. The dependence of the mutual separation factor for the components on their molecular weights difference is obtained. Some aspects of the given model of gas fluid are also discussed.