Eleven types of coals with different swelling property on a rapid pyrolysis were heat-treated below 1500 degrees C in a drop tube furnace (DTF), and investigated their microscopic structure by XRD measurement and TEM observation. XRD patterns of the samples were used to estimate the average number of stacking aromatic layers in a crystallite (N-ave), the stacking index (SI) and the average interlayer distance (d(002)). Dependence of these parameters upon heat-treatment temperature showed that N-ave for every sample had a maximum around 800 degrees C. Amount of change in N-ave from room temperature to 800 degrees C corresponded well to the swelling ratio in the same temperature range, i.e., coal samples with large swelling ratio were likely to have developing stacking structure. Considering that coals generally swell at a plastic stage, the large variation of N-ave in the swollen chars could be related to the increase of mobility of aromatic layers and their local orientation due to the van der Waals effect. TEM observation of swollen chars illustrated that particle walls consisted of isotropic texture, implying that the possible increase of internal pressure when the particle swells had little effect on the development of stacking structure. (C) 2006 Elsevier Ltd. All rights reserved.