Radial artery cannulation is a well established clinical technique for continuous monitoring of arterial blood pressure and serial sampling of arterial blood in critically ill patients. This method is regarded generally as safe, but complications are reported uncommonly. We report a case of false aneurysm of the radial artery following repeated puncture and catheter indweling for 8 days, and discuss briefly about the complication of arterial cannulation. It should be remembered that in arterial cannulation, we must use aseptic techniques, and the catheter must not be indwelt at one point for a long time.