The factors affecting the ignition temperatures of two low-rank coals were experimentally studied using thermogravimetric analysis. The experiments were conducted with coal powders of four different particle size distributions. The thermogravimetric analyzer was operated at three heating rates, 10, 20, and 30 °C min−1 and four oxygen concentrations of 3, 6, 9, and 12%. The results showed that the ignition temperature decreased by about 25 °C as the oxygen concentration increased from 3% to 12%. The standard deviation of the activation energy was 16.75% at a conversion degree of less than 0.4, and it decreased to 1.35% at the end of the combustion process. At a heating rate of 10 °C min−1, the ignition temperature increased by about 8 °C as the coal particle size increased by 100 μm. At a heating rate of 30 °C min−1, the effect of the particle size on the ignition temperature was enhanced and the ignition temperature increased to 15 °C.