With the aim at understanding the self-ignition mechanism of low-rank coals, low-temperature oxidation behaviors of three coals were investigated by measuring the weight change, gas formation rate, heat generation rate, and change of functional groups during the oxidation at the heating rate of 5 K/min in a helium stream containing 22% oxygen. Detailed examination of the initial stage of oxidation below 200 degrees C that will be closely related to the self-ignition clarified the following: the reactions occurring are the intake of oxygen into aliphatic carbons as peroxides and the decomposition of the peroxides forming carboxyls and H2O. For two low-rank coals tested, the former process was rapid enough and only the latter decomposition reaction of the peroxides to form carboxyl groups was observed, which resulted in the monotonous weight decrease and the H2O production. For a high-rank coal tested, the former process solely occurred below 140 degrees C and the former process was faster than the latter process below 250 degrees C, resulting in the monotonous weight increase. The amount of oxygen involved in these processes was found to be only 1.7-4.6 mol of O/100 mol of C of coal. The amount of heat generated during these processes was large enough to raise the coal from 180 to 320 degrees C.