The mechanism fracture behavior around a crack tip in an adhesive layer is important in relation to the strength or adhesive structures. The fracture toughness of a crack tip in a ductile adhesive layer for an adhesive joint depends on the thickness of adhesive layer. However, the mechanism of its thickness dependence has not yet been elucidated. From the viewpoint of microstructure, the effect of thickness of adhesive on the fracture toughness of adhesive joint was investigated. The damage zone and rubber particles dispersed around a crack tip in the adhesive layer of an adhesive joint was observed by an optical microscope. The distribution of the damage zones is observed along th interfaces between the adhesive layer and the two adherends. The results show that the combination of the damage zone around a crack tip and the additional damage zones is th main cause of the variation of the fracture toughness of an adhesive joint with the thickness of adhesive layer.