The main aims of this work are to investigate the constraint effect of adherends on the fracture behaviour of adhesive joints and to study the fracture properties of aluminium-adhesive joints using a rubber-modified epoxy resin (GIc = 2.76 kJ/m2) as an adhesive material. Compact tension (CT) adhesive joints were manufactured for a wide range of bond thickness and fracture tests were conducted. The fracture energy was found to increase, though not monotonically, towards the fracture energy of the bulk adhesive as the bond thickness was increased. The constraint and stress triaxiality at the crack tip in the adhesive joints were also characterised for various bond thickness using elastic-plastic finite element method. It was found that as the adhesive bond thickness was increased, the stress triaxiality near the crack tip was relieved by the intensive deformation of the adhesive. Furthermore, the relationship between J-integral and crack-tip opening displacement (CTOD) was dependent on the constraint around the crack tip. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology. Brittle fracture mechanisms were observed for thin bonds (0.04 < t < 0.5 mm) but tough fracture mechanisms were identified for thick bonds (t > 1 mm). The fracture energy of adhesive joints was proportional to the size of the crack tip plastic zone and the transfer of the bulk adhesive fracture toughness was governed by the constraint at the crack tip. Results from this work would improve integrity assessment of engineering adhesive joints.