The decomposition of carbon dioxide has been investigated using thermal plasma. Carbon dioxide is decomposed in the thermal plasma process of high temperature and converted to another materials in the quenching process. From the thermodynamic equilibrium calculations, it is known that carbon dioxide is decomposed over 2,500K at atmospheric pressure and carbon dioxide is reacted with hydrogen, methane or water to be converted to other material over 1,200K at atmospheric pressure. When carbon dioxide is decomposed directly, the decomposition efficiency is very low because of recombination into carbon dioxide during the quenching step. However when carbon dioxide is reacted with hydrogen, methane or water to be decomposed, the decomposition efficiency is very high because carbon dioxide is decomposed to be converted into carbon monoxide and hydrogen during the quenching step.