The objectives in the present study are to elucidate the fundamental gasification characteristics of woody biomass with a mixture gas of CO2 and H2O. The effects of temperature and gasification agents on the gasification characteristics of woody biomass were experimentally studied using an electrically heated drop tube furnace. The co-gasification performance of CO2 with H2O was compared with the CO2 or H2O gasification performance alone. As a result, H-2 and CO concentrations increased with an increase of temperature under both the co- and single-gasification conditions. CO under the co-gasification condition was produced more than under the single-gasification condition. The H-2 formation showed the opposite tendency to the CO formation during the co-gasification. This synergistic effect is caused by the difference of the surface structure of the woody biomass particles by CO2 from H2O.