To explore the microstructure and oxidation reactivity of crushed coal, the free radicals and functional groups of coals with different ranks in the crushing process were analyzed based on electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectrometry. The low-temperature oxidation of different sized crushed coals was also researched with the aid of a self-built experimental system. The results show that there exists a critical particle size range in the promotion process of the oxidation reactivity of coal. The range is 0.180-0.224 mm for lignite and 0.125-0.224 mm for anthracite. After the coal is crushed to 0.180-0.224 mm, the crushing process begins to exert a weaker effect on the consumption of aliphatic hydrocarbon groups. For the -OH group, the slight increase in the range of 0.180-0.280 mm almost never occurs in the low-rank lignite but is present in bituminous coal andanthracite. Moreover, the rate of C=O consumption in the reaction is higher than its rate of generation, whereas for the product C-O, the rate of consumption is lower than the generation rate. The free radical concentration of oxidized coal is greatly affected by the crushing process. In the particle size range of 0.074-0.125 mm, low-temperature oxidation has the weakest effect on free radical concentration. The oxidation and crushing effects are both strongest for the minimum particle size of 0-0.074 mm. (c) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.