The propensity for spontaneous combustion of Inner Mongolia lignite irradiated in microwave irradiation was investigated. The functional groups on the lignite surface were obtained by FFIR and XPS spectra, and the pore structures of irradiated lignite were measured using N-2 adsorption/desorption method. The propensity for spontaneous combustion of lignite was classified using the crossing-point method. The irradiation treatment in microwave resulted in a prominent decrease in moisture content, decomposition of oxygen-containing functional groups, breakage of long alkyl side-chains, and increase in aromatic carbons. The quality of lignite developed towards high-rank coals after microwave irradiation. Microwave irradiation showed significant effects on the pore structures of lignite; the specific surface area and total pore volume of the irradiated lignite initially increased and then significantly decreased. A short-term microwave irradiation resulted in an increase the spontaneous combustion propensity of lignite. However, when the irradiation time extended, the propensity for spontaneous combustion was reduced because of the significant change in chemical composition and drastic decomposition of the pore structures. (C) 2016 Elsevier Ltd. All rights reserved.