This paper describes a method for preparing metallic Cu nanoparticles in aqueous solution, and a bonding technique using the nanoparticles. Preparation of the Cu particle colloid solution was performed in water at room temperature in air using a copper source (0.01 M CuCl2), a reducing reagent (0.1–1.0 M hydrazine), and stabilizers (0–1.5 × 10−3 M citric acid and 5.0 × 10−3 M cetyltrimethylammonium bromide). The metallic Cu nanoparticles with a size of 71 ± 14 nm were prepared at 0.4 M hydrazine and 5 × 10−4 M citric acid. A stage and a plate of metallic Cu were successfully bonded under annealing at 400 °C and pressurizing at 1.2 MPa for 5 min in H2 gas with help of the metallic Cu particles. A shear strength required for separating the bonded Cu substrates was as high as 28.6 MPa.