<p>This paper provided atomistic insight into the effect of silane coupling agents (SCAs) on strengthening epoxy adhesion to aluminum. The relationship between molecular structure of three different SCAs and fracture modes was elucidated through first-principles calculations. The interfacial bonding strength between Al and SCA was relatively small for aminosilane (APS) and methacryloxysilane (MPS), and was larger and stronger than the bonding strength within epoxy adhesive for epoxysilane (GPS). In addition, the relative relationship of cohesive energy made it possible to distinguish the dominant fracture mode. In tensile loading tests, the adhesive strength increased by 66% for APS, 89% for MPS, and 98% for GPS compared to the untreated specimen, and all SCAs were effective. The validity of the calculation results was verified through tensile loading tests. A cohesive fracture index based on cohesive energy was proposed, and its effectiveness was demonstrated in clarifying the incidence rate of fracture modes.</p> Graphical abstract <p></p>