<p>Cured epoxy resin (CER) adhesives are known to have excellent properties due to their strong strength and ability to interact with adherends. The high chemical cross-linking density structure of CERs plays an important role in their properties. In this study, a low cross-linking density cured epoxy resin (CER-L) was prepared with diglycidyl ether bisphenol A (average degree of polymerization (<i>n</i> = 1.1) (EPON) and phenyl ethyl amine (PEA)), and a high cross-linking density cured epoxy resin (CER-H) was prepared with diglycidyl ether bisphenol A (<i>n</i> = 1) (DGEBA) and 4,4’-diaminodiphenyl methane (DDM). Then, the mechanical properties of the bulk and adhesive properties of the single-lap joint (SLJ) were investigated, and the internal structure was evaluated. The resulting CER-L and CER-H showed similar Young’s moduli and yield stresses, whereas the elongation at break of CER-L was ten times larger than that of CER-H. The shear modulus and strength of the SLJs with CER-L were also comparable to those with CER-H. The failure modes of the SLJs with CER-L and CER-H were cohesive failure and interfacial failure, respectively. In situ small-angle X-ray scattering measurements at various positions in the adhesive during deformation revealed that crazes formed in CER-L during both uniaxial deformation and shear deformation and that plastic deformation occurred at the submillimeter scale. The stress concentration portion at the edge of the adhesive in the SLJ sample was relaxed by the plastic deformation of CER-L; this resulted in a comparable adhesive strength with CER-H. These novel results showed excellent adhesive strength even for the SLJ with CER-L and could be used to provide new guidelines for the design of adhesive materials.</p>