Interfacial fracture toughness under various mixed-mode loading is measured to provide a mixed-mode fracture criterion of a composite/metal bonded joint. Experimental fracture characterization tests were carried out using a SLB (single leg bending) specimen, which controls mode ratio with the specimen thickness. The experimental result of the SLB test conforms that interfacial fracture toughness increases as the mode II component increases. The effect of loading mode on interfacial crack growth is investigated on the basis of crack path observation using microscopic image acquisition technique. The influence of interfacial roughness on adhesion strength is also discussed.