Two competitive processes, i.e. decomposition and condensation, exist in coal pyrolysis. Although a number of approaches have been used to investigate its mechanism, the ex-situ analysis methods cannot reveal the real pyrolysis process because of the complexity of coal pyrolysis. In the present study, the evolution characteristics and mechanism of the aggregate structure of four low-rank coals during pyrolysis process were investigated by the combination of in-situ X-ray diffraction (XRD) and thermogravimetry coupled with mass spectrometry (TG/MS). All the four samples had similar pathways of structural evolution during heat treatment up to 900 degrees C, which can be divided into four distinct stages based on the X-ray structural parameters (d(002), L-c, L-a). The relationship between the structure evolution of coals and gas generation during pyrolysis was investigated. Stage 1 was characterized by obvious increase of d(002) and decrease of L-c and L-a, indicating the imperfection of the aggregate structure due to the swell of small molecules in the interlayer stacking structure and the broken of the hydrogen bonds. Minor changes of these parameters in stage 2 demonstrated the stability of aromatic layers during the main pyrolysis stage. Stage 3 was identified by the distinct decrease of d(002) and increase of L-c and L-a, reflecting the development of aggregate structure, which is accompanied by the evolution of H-2. In the last stage, d(002), L-c and L-a showed minor changes. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.