A high-concentration colloid solution of metallic copper nanoparticles was prepared by the reduction of copper ions in aqueous solutions, and the nanoparticles were used to study metal–metal bonding. The colloid solution of metallic copper nanoparticles was prepared by mixing an aqueous solution of CuCl2 and an aqueous solution of hydrazine (reductant) in the presence of citric acid and n-hexadecyltrimethylammonium bromide (stabilizers) at a final copper concentration as high as 0.1 M. The copper-particle size was ca. submicrometer. Many tiny particles with sizes of ca. 10–20 nm were deposited on the submicron-sized particles at high hydrazine concentrations. The tiny particles were considered to be produced through the growth of particle nuclei generated at the final reaction stage by residual hydrazine derived from the high hydrazine concentration. Metallic-copper discs were successfully bonded by annealing at 400 °C and a pressure of 1.2 MPa for 5 min in hydrogen gas with the help of the metallic-copper particles. The shear strength of the bonded copper discs was then measured. The highest shear strength of 41.7 MPa was recorded for the tiny particles/immobilized particles, which could be explained by the low melting points of the tiny particles and the increase in bulk density provided by the dominant incorporation of the tiny particles into voids between the submicron-sized particles.