The "reactivity" of coal is one of the important characteristics of a coal used as a process raw material as well as a fuel. In this study, the reactivity was measured in terms of the magnitude of the reaction rate constant in the reduction of carbon dioxide with coal. A reactivity-testing apparatus was designed and constructed, and its performance characteristics were investigated by using Korean anthracite and hard-wood charcoal. Experiments were carried out at temperatures ranging from 750 to 1100$^{circ}C$ with pulverized Korean anthracite whose sizes range from 1 to 10mm in diameter. Results showed that the reaction rate constant was not appreciably affected by the particle size investigated, and the reactivities of the anthracite and the charcoal were found to be a function of reaction temperature alone. It was also found that a straight line was produced when the logarithm of the rate constant is plotted against the reciprocal of the absolute temperature. The reactivities of the charcoal were found to be 2 to 10 times higher than those of the anthracite at a temperature ranging from 750 to 1100$^{circ}C$, and 90% of carbon dioxide was reduced to carbon monoxide by the anthracite at a temperature above 1050$^{circ}C$.