Bonding metal with composite materials by means of the emerging high strength structural adhesives is the ultimate solution in order to keep up with the optimization of the automotive industry regarding the lightness and safety of the vehicles. The lack of knowledge about the fracture mechanics of these joints is yet the main disadvantage when considering their implementation into demanding structural applications, such as in crashresistance components. The present study has investigated the fracture toughness of the joint between carbon fiber reinforced polymer (CFRP) and boron alloyed high strength steel in modes I and II of failure according to the theory of linear elastic fracture mechanics (LEFM). The substrates were bonded together using a toughened structural epoxy adhesive developed for the automotive industry. Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests were adapted to the requirements of the system. The results show high critical energy release rates for both crack propagation modes. This proved the strength of the adhesive joints and its reliability for this kind of applications rather than traditional or welded joints, which would induce severe damages to the CFRP substrate. In mode I of failure, the CFRP/adhesive interface was the weakest. This explains why the comparison between the two sets of joints that used either uncoated or Al-Si coated steel didn’t result in significant differences. An important upgrade is observed when comparing the obtained results to a previous study, which used a co-cured bonding method without additional adhesive layer between the substrates.