The arterial cannulation has been performed for continuous blood pressure monitoring and blood gas analysis in operating room. The radial artery, ulnar artery, dorsalis pedis artery and others have been used for the site of cannulation. Although arterial cannulation offers many advantages, complications can developed. The complications of arterial cannulation are pain on puncture site, hematoma, infection, and so on. However pseudoaneurysm is a rare complication. We experienced a case of pseudoaneurysm at the radial artery following catheter indwelling for 7 days in a 54-year-old male patient undergoing cerebral aneurysm clipping which was found at 84 days after operation.