Three　sets　of　uniaxial　tensile　tests　have　been　performed　in　situ　in　transmission　electron　microscopy/high-resolution　electron　microscopy　on　Cu　nanowires　(NWs)　to　accurately　map　out　the　sample　size　dependence　of　elastic　strain　limit.　Atomic-resolution　evidence　was　obtained　for　an　exceedingly　large　recoverable　strain　(as　much　as　7.2%)　that　can　be　sustained　in　the　lattice　of　a　single-crystalline　Cu　NW　with　a　diameter　of　similar　to　5.8　nm.　This　ultrahigh　elastic　strain　is　consistent　with　the　predictions　from　molecular　dynamics　simulations　for　nanowires　and　approaches　the　ideal　elastic　limit　predicted　for　Cu　by　ab　initio　calculations.