The thermodynamic equilibrium calculations of the direct decomposition of CO₂, the reaction of CO₂ with CH₄, the reaction of CO₂ with a mixture of CH₄ and O₂, and the reaction of CO₂ with H₂ were performed in the temperature range of 500k ∼6,000K at atmospheric pressure. The direct decomposition of CO₂, the reaction of CO₂ with CH₄, the reaction of CO₂ with a mixtureof Ch₄ and O₂ and the reaction of CO₂ with H₂ were studied using thermal plasma as a heating process and a water-cooled tube as a quenching process. The typical plasma power was 4.8kW at atmospheric pressure. The conversion into carbon monoxide of the direct decomposition of carbon dioxide was found less than 11%. The conversion of the reaction of carbon dioxide with reactant gas(CH₄ or H₂) was much higher than the conversion of the direct decomposition of carbon dioxide. The effects of CH₄ on the conversion of CO₂ are better than the effects of H₂. The maximum conversion obtained in this work was 89%.