Cu and Al6061 alloys were diffusion-bonded at 530°C with Cu-15 wt. % Ag alloy as an intermediate layer and the interface microstructure were analyzed. The presence of Cu9Al4, CuAl and CuAl2intermetallic layers were confirmed along with Ag3Mg and AgMg intermetallics. Ag was found to diffuse into Al, forming needle-shaped Ag2Al precipitates in Al substrate close to Al/Cu-Ag interface region. Micro-Vickers indentation aimed at the interface regions, CuAl/CuAl2and CuAl2/Ag3Mg-AgMg lamella revealed that cracks were formed mostly around the indenter mark in CuAl2layer more than in CuAl and no cracks in the Ag3Mg-AgMg lamella region, suggesting CuAl2is more brittle than CuAl and Ag3Mg-AgMg lamella is rather ductile. Ag3Mg-AgMg lamella was found to be formed at the interface between Al and CuAl2. The formation of ductile Ag3Mg-AgMg lamella at the Al interface suggests that the diffusion bonding Cu and Al with Cu-Ag or Ag as a intermediate layer may increases the interface reliability of Cu/Al hybrid alloy.