The mechanisms of ignition of coal of different quality, such as anthracite, bituminous coal and lignite, were studied by thermogravimetry (TG) and differential thermal analysis (DTA). Experiments on coal ignition were carried out at a low heating rate (10 K min(-1)) with particle sizes ranging from 37 to 4000 mu m. It is concluded from this experimental work that (1) ignition measurement by TG-DTA is an excellent method for accurate determination of the ignition temperature of coal particles; (2) with increasing coal quality from lignite through bituminous coal to anthracite, the type of ignition changes from homogeneous ignition through hetero-homogeneous ignition to heterogeneous ignition, and the ignition temperatures also increase; (3) with increasing coal particle size, the type of ignition of Kaipin bituminous coal changes from hetero-homogeneous to homogeneous ignition, and ignition of the char separates from that of the volatile matter and shifts to a higher temperature, whereas both types of ignition of Loy Yang lignite coal and Hongay anthracite coal are not effected by particle size.