Orbital blowout fracture in facial fractures is the most difficult to handle and mechanism of the fracture is unknown. Only one experimental proof is made by Smith, who insists of the theory of increased hydraulic pressure in the orbital contents as a causative factor. Our clinical cases do not always present such an evidence. In order to clarify the mechanism, the serial tests were designed. The first test is stress coat test, which suggests that stress concentration is quantitatively focused along the suture lines of the orbital walls and the infraorbital canal. The phenomenon is notch effect.