Octahedral and hexahedral diamond particles in the size range 4-450 mum were formed at temperatures of 400-600 degreesC by reduction of dense carbon dioxide with metallic lithium and potassium, respectively. The micro-Raman spectrum analysis shows that typical particles exhibit a sharp peak at 1332 cm(-1), which is the characteristic signature of cubic diamond. It is found that much larger diamonds can be yielded by using metallic potassium as reductant than that by using metallic lithium, which is explained in terms of the difference in molten characteristic between the two alkali metals.