Treatment of biomass in nitrogen and air at 240-340 degrees C was conducted in order to examine the low-temperature region in a downdraft gasifier by analyzing the treated precursors and product distribution, Gas-treated precursors were then pyrolyzed in flash mode at 764 degrees C for further analysis. Overall, the tar yield decreased from approximately 50 wt % to less than 20 wt % upon oxidation of the sample at a very low heating rate to 260-300 degrees C in air. Moreover, tar evolution was almost completely suppressed during the subsequent flash pyrolysis. This indicates that the structure of the treated precursors was gradually changed to suppress tar release through cross-linking reactions and partial oxidation. From elemental analysis of the precursors treated with air, it was also estimated that dehydration and partial oxidation proceeded simultaneously. The results indicate that the release of tar products such as dimers call be partially suppressed by air treatment at low temperature.