Abstract: 3 low rank coals were dewatered and upgraded by hydrothermal process at 200, 250 and 300 ℃. Nitrogen adsorption measurements and FT-IR spectra of the parent and upgraded coals were performed to get the change of pore distribution and functional groups. After hydrothermal treatment, the average pore diameter and specific pore volume drop rapidly. The content of hydroxyl and carboxyl decreases and the cyclized degree increases. The monocyclic aromatics change into polycyclic aromatics and the organic matter in coal is gradually mature, which reflect the coal rank becomes higher. Then, rapid pyrolysis were carried out in a CDS5000 Pyroprobe accompanied by a GC/MS at 5 000 ℃/s, 1 000 ℃ to investigate the release characteristics of macromolecular volatile products of these coals. With the final reaction temperature increasing, the content of aliphatic hydrocarbon, acids and phenols decreases, that of aromatic hydrocarbon increases. The esters in coals have no obvious variation law.