Cu alloy has been suggested to enhance reliabilites in future technology. To evaluate the effects of alloying elements on electrical and mechanical reliabilities, Mg and Ru were chosen as alloying elements. Because alloying elements to Cu should not increase resistivity over comparable value to pure Cu, the resistivity of pure Cu, Cu-0.7at%Mg alloy and Cu-2.2at%Ru alloy was compared. Time dependent dielectric breakdown (TDDB) under bias-temperature stress (BTS) tests were accomplished to evaluate the effect of alloying elements on dielectric reliability. The standard four point bending experiments were conducted to measure critical interfacial adhesion energy of conducting metal to SiO2. The relationship of the effect of alloying elements on reliabilities to distribution of alloying elements was analyzed using Auger Electron Spectroscopy (AES). Our results show alloying Mg to Cu can enhance reliability by forming self-passivating layer on its surface and interface while Ru does not.