Abstract: It is shown that the probability of an disordered atom to be ordered in unit time can be correlated to the coefficient of diffusion by the relation a=9/4 d/a2 (m2/m1)1/2 (△S2-△S1)/k(Tc/T-1) where a=the probability of ordering, D=the coefficient of diffusion, a =the lattice constant of AuCu3,m1 and m2 the mass of an atom of gold and copper respectively,△S1, △S2 being respectively the entropy change when a gold and a copper atom jumps to a neighboring vacancy, k, the Boltzmann constant, Tc, T, the critical temperature and the absolute temperature under consideration.This relation has been verified with experimental data. With experimental value of a and D0, it gives an activation energy of 2.10 eV which is equal to the activation energy of self-diffusion of copper within the limits of experimental accuracy. This expression explains the existance of a temperature at which the rate of ordering is maximum both qualitatively and quantitatively.