Abstract: Based on 8 active groups leading to spontaneous combustion of coal, a simple model of coal was developed to investigate the reaction mechanism of spontaneous combustion of coals at initial stage. Calculations about coal molecules model were carried out with Gaussian 03 (DFT-B3LYP) program. The results show that the oxidation reactions of 1, 3, 5, 6, 7, and 8 are exothermic, and except for reaction 2 and 4, other reactions have negative ΔGθ, which means that these reactions are spontaneous at room temperature. According to the calculations of activation energies of the reactions, it is concluded that six among the eight reactants tend to react with oxygen spontaneously at room temperature. Furthermore, the order of activity of these active groups deduced during experiments is roughly confirmed by the theoretical calculations. The reaction mechanism of low-temperature oxidation of coal at initial stage is also presented. The results well agree with those obtained by previous experiments.