In order to supply the syngas (CO + H2) to the liquid fuel synthesis reaction in the biomass-to-liquid (BTL) system, woody biomass gasification was carried out using a mixture of CO2 and O2 as a main gasifying agent. Gasification of biomass using a CO2/O2 mixture (79/21(v/v)) exhibited a maximum value of the conversion to gas (88 C-mol%), compared to those (78 C-mol%, 80 C-mol%) using pure CO2 and air, respectively The effects of [H2O]/[C] ratio and O2 content on the gasification behavior with CO2/O2 were investigated. The H2/CO ratio in the product gas increased from 0.3 to 1.0 due to the promotion of water-gas shift reaction when the [H2O]/[C] ratio was increased in the range of 0 - 5. The conversions to gas and tar increased from 88 to 94 C-mol% and from 0.1 to 0.6 C-mol%, respectively; on the other hand, conversion to char decreased from 8.7 to 0.7 C-mol%. The CO2 and H2 contents increased from 53 to approximately 58 mol% and from 11 to 19 mol%, respectively; however, the CO content decreased from 32.3 to 18.1 mol%. The conversion to gas increased with the O2 content (0 - 50 vol%) in the gasifying agent, while the conversions to tar and char decreased from 0.8 to 0.0 C-mol% and from 16 to 4.1 C-mol%, respectively. The CO and H2 contents increased from 36 to 42 mol% and from 9.5 to 20 mol%, respectively, while the CO2 content decreased from 49 to 32 mol%. In all the cases, H2S and COS were detected in the product gas. The conversions to gas on a sulfur basis were approximately 60 S-mol% at all [H2O]/[C] ratios. However, with an increase in the [H2O]/[C] ratio, the conversion to H2S increased from 38 to 46 S-mol%, while that to COS decreased from 25 to 16 S-mol%; this was attributed to the occurrence of the following reaction: COS + H2O → H2S + CO2. With an increase in the O2 content, the conversion to H2S increased from 38 to 44 S-mol%, while that to COS decreased from 25 to 18 S-mol%. The addition of O2 to the gasification with CO2 promoted the gasification of tar and char to produce H2S than COS regardless of the oxidation atmosphere. From the results obtained, the BTL process including the woody biomass gasification with a CO2/O2 mixture, dry gas cleaning, water-gas shift reaction, removal of CO2 and liquid fuel synthesis reaction was proposed.