Epoxy-based adhesives are widely used for repairing or jointing the metal sheets in the industry. Because of their superior mechanical properties, the metallic nanoparticles can be selected as the additive of the epoxy adhesive. The strength of the Cu nanoparticles (CuNPs) can be expected to improve the mechanical properties of neat epoxy. In this study, CuNPs were added at various weight ratios, such as 1, 2, 5, 10, 15, and 20% into the epoxy resin adhesive. Tensile tests of the dog-bone specimens and the lap-shear tensile tests of the single lap joints were performed for obtaining the mechanical properties. In order to investigate the failure mechanisms, the fractured surfaces of the tensile test samples and adhesively joined sheets were imaged by using a Scanning Electron Microscope. The thermal properties of the adhesives were obtained by using Thermo Gravimetric Analysis and Differential Thermal Analysis. The mechanical and thermal properties of epoxy resin adhesive were improved by adding the CuNPs. The best adding ratios of CuNPs into epoxy were obtained by both mechanical and thermally point of views. As a result of this study, 15 wt% the ratio of Cu nanoparticle adding into the epoxy-based adhesive is suitable for improving the mechanical properties. On the other hand, 20% is the proper Cu nanoparticle adding ratio for the thermal properties improving.