The synergetic effect in reactivity and gas yield on the various ratio of CO2/steam mixtures was investigated. The isothermal gasification was conducted at three different temperatures. The synergy effect was evaluated on the ratio of CO2/steam mixtures and reaction temperatures. In order to analyze the synergy quantitively, two reaction indexes were calculated from carbon conversion. The effect of natural minerals like Dolomite and Kaolin was investigated as well. The influence of synergy was varied upon the ratio of CO2/steam mixtures and the optimal synergy was observed when the ratio of CO2/steam mixtures was 1:2. The best synergy in reactivity and gas yield was shown at 800 °C and at 900 °C, respectively. By adding Dolomite, the synergetic effect in both reactivity and H2 yield was promoted at 800 °C. Conclusively, the ratio of CO2/steam mixtures and Dolomite played an important role to facilitate the synergy in the coal gasification.