Eighteen kinds of varying rank coals were gasified with steam at 1185°Cby use of a rapid heating method to clarify the factors which control the gasification reactivity of the coals. With the aid of the gasification data of demineralized coals, the gasification rate of lower rank coals were found to be, in part, catalyzed by coal mineral matters. The reactivity of higher rank coals and the demineralized coals were well correlated with both the pore surface area and the crystallite size of carbon. This was supposed because the increase of crystallite size results in the decrease in the pore surface area and the number of active sites. Furthermore, the gasification rate increased with the amount of oxygen trapped by the char, which was measured by flash desorption method. This showed that the amount of oxygen trapped is intimately related to the reactivity even at the high temperature.