In this study, effects of the operational parameters such as gasifier temperature, bed operational velocity, equivalence ratio, biomass particle size and biomass-to-steam ratio on hydrogen production from an atmospheric biomass FB gasifier is simulated by presently developed model. The model is one-dimensional, isothermal and steady state, and the fluid-dynamics are based on the two-phase theory of fluidization. Tar conversion is taken into account in the model. The model simulation results are also compared with and validated against experimental data given in the literature. As a result of this study, it is observed that H-2 composition increased remarkably with the rise of the gasifier temperature. Small biomass particles improves H-2 composition. It is unfeasible to apply too small or too large ER in biomass air steam gasification. The increases in the mole fractions of H-2 with increases in the steam flow rate indicated that the gas shift reaction has a substantial effect in air steam gasification. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.