Ultrahigh　hardness　(yield　strength)　was　achieved　in　magnetron　sputtering　nanoscale　Cu/Al　multilayers　upon　annealing.　The　microstructure　and　mechanical　properties　of　the　multilayers　were　systematically　investigated　by　X-ray　diffraction,　transmission　electron　microscopy,　energy　dispersive　X-ray　spectroscopy　and　nanoindentation.　Annealing　promoted　diffusion　of　Cu　and　Al　atoms　in　the　interfaces　and　the　sharp　interface　turned　to　mix,　resulting　in　the　formation　of　Cu/Al　intermetallic　compounds　and　its　deformation　at　nanoscale.　The　Cu/Al　intermetallic　compounds　mainly　including　Al2Cu　grew　toward　to　Al　layers　and　would　reducing　the　effective　length　between　the　reduced　adjacent　layers.　As　the　annealing　temperature　was　increased　from　100　degrees　C　to　500　degrees　C,　various　kinds　and　larger　size　Cu/Al　intermetallic　compounds　emerged,　causing　the　hardness　to　first　increase,　reaching　an　unusually　high　peak　(never　reached　before　in　other　thin　metallic　multilayer　systems),　and　then　remain　nearly　unchanged.　The　physical　mechanisms　underlying　such　remarkable　enhancement　were　explored　in　terms　of　interface　alloying,　reduced　layer　thickness　and　grain　size　effects.