The sawdust tar was generated from air-steam gasification in the fluidized bed reactor at atmospheric pressure. The gasification temperature and mass ratio of steam to sawdust were used as adjustable variables with which the amount of sawdust tar formation and the corresponding tar properties. Gel permeation chromatography (GPC) coupled with diode array detector (DAD) was employed to analyze the tar. It was found that the molecular weight distribution (MWD) of tar was unchanged under investigated conditions. The majority of the tar components are aromatic compounds, paraffin with conjugated bonds and olefin. Tar molecular weight (MW) depends on the H free radical amount, which is related to the steam added during the gasification. The steam can prevent the polymerization reaction and more conjugated side-chain compounds would be formed. In the end, the sawdust tar formation process has been described from the tar structure obtained. (C) 2009 Elsevier Ltd. All rights reserved.