The upgrading of high-temperature coal tar in supercritical water (SCW) was investigated using an autoclave reactor. The effects of temperature (673-753 K), pressure (24-38 MPa) and residence time (1-80 min) on product distribution were studied. The yields and characteristics of products in SCW were compared with those obtained in N-2. In order to study the reaction mechanism of coal tar upgrading, the experiments of model compounds were carried out in SCW at 673 K and 34 MPa for 20 min. The results indicate that the asphaltene conversion and the maltene yield are significantly higher in SCW. The H/C atom ratios of oil obtained in SCW are higher than those obtained in N-2. More valuable aromatic compounds such as naphthalene, fluorene and anthracene are obtained in SCW. The results of model compounds suggest that water in SCW could participate in coal tar upgrading and promote the formation of light products. (C) 2008 Elsevier B.V. All rights reserved.