The change in the rate of discontinuous precipitation in Ag4Cu alloy has been observed with ternary element Sn added. The reaction rate gradually decreases with the ternary Sn addition and does not occur at Ag-4Cu-8Sn composition while chemical free energy change for precipitation still exists. This reaction again occurs and its rate increases with the ternary Sn addition over 8%. The faceting phenomenon which indicates that coherency strain energy is the driving force for this reaction is observed. The change of this reaction rate is explained in terms of coherency strain energy. Furthermore the guide line for the choice of ternary element is presented under the assumption that precipitation may occur initially in equilibrium.