In this study, PVP-protected Cu@Ag core-shell nanoparticles with different Ag shell thicknesses were synthesized and applied to Cu to Cu bonding. The phase transformation of Cu@Ag core-shell nanoparticles (about 80 nm in average particle diameter and 10 nm in shell thickness) upon heating in air was investigated via in situ synchrotron radiation X-ray diffraction. The formation temperatures of copper oxides (CuO and Cu 2 O) were higher than those of pure Cu nanoparticles. This verifies that the oxidation of Cu nanoparticles can be effectively retarded by the Ag shell. It is also demonstrated that firm bonding between two Cu pads could be obtained using Cu-Ag particle pastes after thermal compression at 250 °C.