Radial artery is the most common site which is chosen for the continuous monitoring of arterial blood pressure and blood sampling in the critically ill patients. This method is regarded as a safe one though there are various complications. In most complications, the clinical sequelae do not have much significances. However, serious complications have been reported including cerebral embolism, or ischemic necrosis of hand or forearm requiring amputation although the incidences are rare. Post-cannulation radial artery aneurysm is not common in clinical practices. We report a case of post-cannulation radial artery aneurysm in a 23-year-old female patient undergoing mitral valvular replacement. Radial artery cathteter was indwelt for 14 days at the same site. It should be kept in mind that aseptic and atraumatic technique must be used and the catheter must not be indwelt at one point for a long time in arterial cannulation.