The effects of Palladium addition to stoichiometric AuCu on the age-hardening are studied by means of resistivity, hardness, X-ray diffraction, and optical and electron microscopic observation. The results obtained are summerized as follows. The shapes of the age-hardening curves in the alloys less than 23.4 at. % Pd, showed an abrupt increase at the early stage of ageing. However, in the alloy containing 34.4 at. % Pd, the age-hardening was not observed. The maximum hardness of the AuCu-12 at. % Pd alloy was higher than that of the AuCu-6 at. % Pd and AuCu-23.4 at. % Pd alloys. The difference in hardening in these alloys was explained by the effect of atomic sites of palladium occupied in AuCu superlattice. Optical and electron micrographs showed that twinning occurs in order to relief the elastic strain induced by growth of AuCu I type superlattice platelets.