This Communication reports a low-temperature diamond synthesis technique, in which diamonds (10-250 mum) can form at a temperature as low as 440 degrees C by reduction of dense CO2 with metallic Na. The X-ray diffraction pattern of a powder sample shows three reflection peaks, indexed with 111, 220, and 311, corresponding unambiguously to cubic diamond. The Raman spectrum of the product exhibits an intense first-order peak at 1332 cm-1, which is the characteristic signature of the cubic diamond, indicating the formation of well-crystallized diamond. Carbon dioxide is a nontoxic low-energy molecule, abundant on earth. This novel reduction method could allow studies of large-size diamond growth using CO2 as the carbon source.