Abstract: The dependence on pH and ion strength for extraction equilibrium of lysine in reversed micelles aqueous system in which ammonium di(2 ethylhexyl) phosphate was used as surface active agent were studied. The main two ionic species of lysine's distribution coefficient were calculated by material balance. Considering the effect of ion strength on the two ionic species' activity, distribution equilitrium dependence on ion strength was established and the parameters were also obtained. Thus, the general distribution coefficient can be calculated easily, the predicted results are in good agreement with experimental data.