In this research, effect of particle size on the oxidation mechanisms of Cayirhan, Tuncbilek and Soma lignites are studied. Non-isothermal thermogravimetry and derivative thermogravimetry (TG/DTG) experiments were carried out for 12 different size fractions. Data obtained from TG/DTG experiments are converted into dimensionless time (instantaneous size/original size) versus dimensionless time (time/burn-out time) to show the progress of oxidation mechanisms of lignites, It was found that lignites show linear behaviour at elevated temperatures, which justifies the assumption that chemical reaction is the controlling step.