She departed from Kiel on 5 December 1944 on her last mission, to transport to Japan a large quantity of mercury, parts, and engineering drawings for German jet fighters. While returning to Bergen, Norway to repair a misfiring engine, the U-864 was detected and sunk on 9 February 1945 by the British submarine HMS Venturer, killing all 73 on board. It is the only documented instance in the history of naval warfare where one submarine intentionally sank another while both were submerged.