This report documents a case of three-channeled aortic dissection. The diagnosis of dissecting aneurysm was made by chest X-P and CT to 70-year-old man, with a chief complaint of back pain. Aortogram showed aortic aneurysm (DeBakey type IIIb), which had an entry at distal of the beginning of the left subclavian artery. Though we had given a pressure control therapy, the patient died on the 5th day of the admission. At autopsy, a new dissection was found in the chronic dissecting outer wall, forming three channeled dissection and rupture was there. Three-channeled dissection is very rare, only 8 cases including ours have been reported so far. From this case, we learned it very difficult to diagnose and treat it.