Abstract: The present work describes synthesis of Ag/Cu composite nanoparticles in aqueous solution, and metal-metal bonding in hydrogen gas or nitrogen gas using the nanoparticles. Synthesis of the Ag/Cu composite nanoparticles was performed by the following process. First, a colloid solution of metallic Cu nanoparticles was prepared by reducing copper ions using hydrazine in an aqueous solution containing citric acid and polyvinylpyrrolidone. Then, metallic Ag nanoparticles were fabricated by reducing silver ions with hydrazine in the presence of the metallic Cu nanoparticles. The metallic Ag nanoparticles with a size of ca. 25 nm were immobilized on the metallic Cu nanoparticles with sizes of 60-120 nm. Cu-Cu bonding was performed by pressurizing metallic copper discs sandwiching the nanoparticles as a filler at 1.2 MPa for 5 min under annealing in hydrogen gas or nitrogen gas. The shear strength, which was required to separate the bonded discs, recorded 26.8 and 12.1 MPa for the bonding in hydrogen gas and nitrogen gas, respectively.