For sustainable development, eco-friendly epoxies, composed of recycled rubber micro-particles and core-shell rubber (CSR) nanoparticles, were processed to improve adhesion strength of an epoxy resin to structural steel substrates. To achieve this, the influence of mixing ratio of a recycled rubber micro-particles and CSR nanoparticles on the adhesion strength and adhesion fracture mechanisms of an epoxy were investigated in the present study. Synergistic adhesion strength was obtained for a certain combination of 7.5 parts per hundred resin (phr) CSR nanoparticles and 2.5 phr recycled rubber micro-particles. The remarkable increase in adhesion strength was attributable to “branching” of the crack-tip damage zone plus crack deflection to cause an adhesive to cohesive fracture transformation.