Direct arterial pressure monitoring by means of an intra-arterial catheter has been considered benefit for assessment of the critically ill patients, safe conduct of controlled hypotension and frequent obtaining arterial samples for blood gas analysis. However in stead of these advantages, there would be high incidence of potential complications of arterial catheterization, such as pain, trauma to the artery and surrounding tissues(e. g., nerve), hematoma, infection, thrombosis, and distal embolization of air, clot, pieces of the catheter, and other debris. We have recently experienced an unexpected episode of amputation of the upper extremity resulting from axillary arterial occlusion following accidental injection of diphenylhydantoin through the radial arterial catheter in 57 year old neurosurgical patient. To prevent these serious and unexpected complications following arterial cannulation, we have to keep a continuous interest and vigilance to those who have invasive monitorings and those who are stranger to handle the arterial cannulation.