In this paper, the effect of adding multi-walled carbon nanotubes (MWCNTs) with an outer diameter of less than 8 nm to an epoxy adhesive was studied on the adhesive fracture resistance and damage behaviour. The fracture energies of the neat and toughened adhesives were measured by testing double-cantilever beam specimens. Moreover, a cohesive zone model (CZM) was used to numerically study the effect of MWCNTs on the damage behaviour of the toughened adhesives. The maximum improvement of 58.4% in the adhesive fracture energy was obtained when the adhesive was toughened with 0.3 wt% of MWCNTs. The fracture surfaces were analysed using the scanning electron microscopy (SEM) technique. It was found that the presence of MWCNTs in the toughened adhesives caused rougher fracture surfaces. Moreover, some fracture mechanisms including nanotube pull-out and de-bonding were observed in the fracture surfaces. The numerical analyses showed that the damage process zone length was also influenced by MWCNTs. The longest damage process zone was obtained for the toughened adhesive with 0.3 wt% of MWCNTs.