We have developed a plating-free bumping technology by using Cu nanopaste and Injection Molded Solder (IMS) method. In this bumping process, Cu pillars are formed by filling Cu nanopaste into photoresist openings and sintering. IMS method is then applied for solder bumping. Cu nanoparticles in the paste coalesce and form necking structure by heating. The characteristics of sintered Cu nanoparticles change depending on the sintering conditions. We have already reported that the grain size of sintered Cu nanoparticles becomes larger, as the sintering temperature increases. In this study, we focus on the mechanical characteristics of sintered Cu nanoparticles, especially on the Young’s modulus and the yield stress. The values of Young’s modulus of Cu nanoparticles sintered at 150oC, 200oC, and 250oC were 56.0 GPa, 67.6 GPa, and 65.3 GPa, and the values of yield stress were 1.00 GPa, 1.09 GPa, and 1.11 GPa, respectively. The results suggest that the mechanical characteristics of the sintered Cu nanoparticles are correlated to the morphology such as grain size and necking structure between Cu nanoparticles.