The effect of the type of gasifying agent used in biomass gasification on product distribution (gas, char and tar yields) and gas quality (contents in H-2, CO, CO2, CH4, ..., tars) is analyzed. Gasifying agents taken into account are: air, pure steam, and steam-O-2 mixtures. Process considered is biomass gasification in atmospheric and bubbling fluidized bed. Previous results got by Herguido et al. (Ind. Eng. Chem. Res. 1992; 31(2): 1274-82), Gil et al. (Energy and Fuels 1997; 11(6): 1109-18) and Narvaez et al. (Ind. Eng. Chem. Res. 1996, 35(7): 2110-20) are compared. Such authors carried their research on biomass gasification under similar conditions but varying the gasifying agent. Three different gasifying agent-to-biomass ratios are;needed and used to compare results. The relationships between the H-2, CO2, ..., tar contents in the flue gas and the type and amount of gasifying agent used are shown after a carefully analysis.