The ignition characteristics of single coal particles under microgravity conditions were investigated by means of numerical modeling, considering the effects of particle size, ambient temperature, and oxygen concentration as well as intraparticle thermal conduction. The predicted particle surface temperature and flame displacement agreed well with the experimental data observed under microgravity. It was revealed that when the coal particle size was greater than 100 mu m, the intraparticle temperature distribution becomes significant. The primary ignition mechanism was heterogeneous for smaller particles but homogeneous for larger particles. The transition diameter for the ignition mechanism to switch from heterogeneous to homogeneous was found to depend on the ambient temperature, O-2 concentration, and whether the intraparticle thermal conduction was considered or not. At a relatively low ambient temperature of 1123 K in air, the predicted transition diameter changed from 800 mu m to 600 mu m when intraparticle thermal conduction was considered. However, at 1500 K, the intraparticle thermal conduction had little effect on the ignition temperature and the transition diameter predicted was about 700 mu m. It was shown that homogeneous ignition occurred when O-2 concentration was low while heterogeneous ignition occurred when O-2 concentration was high. It was also shown that the homogeneous ignition temperature was slightly lower than the heterogeneous ignition temperature. At high O-2 concentrations, the predicted ignition temperature and ignition delay decreased with increasing O-2 concentration, consistent with the literature findings, including those from the authors own previous studies. However, at low O-2 concentrations, the predicted ignition temperature and ignition delay were less variant. The discrepancies of the predicted transition diameters for the isothermal particles and those with intraparticle thermal conduction considered were attributed to the heating rate and the influence of volatile matter release.