Extensive study has been made on orthopyroxenes by X-ray diffraction and Mossbauer resonance technique, and the intracrystalline distribution of Fe2+-Mg2+ over M1 and M2 sites has been determined. Fe2+ ion is strongly preferred at M2 site while Mg2+ at Ml site. Several workers have interpreted the orderly-disorderly kinetics in terms of ideal solution model and simple composite model.In this paper site distribution data are used to calculate the Fe2+ ion energy difference between Ml and M2 sites, by assuming Boltzmann distribution of ions over the sites and the blacking of sites as a result of preferential substitution. From the results calculated it is inferred that there are composition-dependence and temperature-dependence for Fe2+ ion energy difference △E12 between Ml and M2 sites in ortho-pyroxene. According to classical thermodynamics and kinetics Virgo and Hafne have given the value of activation energy E0 = 16.5 Kcal/mole for orderly process and ED = 20 kcal/mole for disorderly reaction, thereof △E = ED - E0 = 3.5 kcal/mole is in good agreement with our results. The value △E12 obtained in this work can also be used to interpret the influence of annealing temperature and compositional changes on relative site distribution.