Carbon spheres with approximate uniform diameters of about 1-2 mu m were synthesized by chemical reduction of supercritical CO2 with metallic calcium at 550 degrees C. The products were characterized by techniques including XRD, scanning electron microscope, Raman, and transmission electron microscopy. The formation mechanism was thought to be as follows: the thermal movement of carbon nanogranules on the surface of metallic calcium leads to the formation of carbon spheres, which was confirmed by reference experiments at different temperatures. What is more, the formation mechanism of carbon sphere in supercritical CO2 system was discussed. (c) 2006 Elsevier B.V. All rights reserved.