A 100 kW fixed bed downdraft gasification reactor has been developed in a joint bilateral project between a Greek and a Romanian research team. The main purpose was to establish the main operational characteristics of the lab-scale unit and to examine the effect of oxygen increase in the gasification air mixture on the quality of the produced fuel gas, concerning combustion behavior. Two different types of biomass types have been used; oak wood saw dust pellets and sorghum pellets. The results demonstrated that when the oxygen content of the gasification air mixture was increased, the combustible content (CO, H2, and CH4) and heating value of the fuel gas was also increased, for both biomass types.