Thermocompression bonding of copper to copper using copper nanoparticles is studied using molecular dynamics. The bonding interface formation process is investigated frst. For the bonding process, the effects of temperature and external pressure are examined. Also, we examine the grain growth at the interface. The results show that the nanoparticles with high surface energy and low compressive strength provide the active atoms to bond with copper. Pressure determining the degree of deformation of nanoparticles transfers atoms from the interior to the surface of nanoparticles and provide more surface atom to form bonds with bulk copper. While continuous pressure increase does not help bonding, higher temperature will facilitate formation of vacancies by breaking the bonds and driving the metal atoms into these vacancies. In addition, a higher temperature promotes grain growth at the interface. These behaviors indicate that using nanoparticles as a bonding layer in metal bonding can effectively reduce bonding temperature and pressure. It is necessary to select appropriate pressure at initial bonding stage and provide continuous high-temperature hold time.