A two-phase model capable of predicting the performance of fluidized bed biomass air-steam gasification reactor during dynamic and steady state operations was developed based on the two phase theory of fluidization. Material and energy balances were taken into consideration and the minimization of free energy technique was used to calculate the gas mole fractions. The fluidized bed was divided into three zones (jetting, bubbling and slugging) and the mass and heat transfer coefficients were calculated for each zone in both bubble and emulsion phases. The model includes the hydrodynamics, transport and thermodynamic properties of fluidized bed. The finite element method was used to solve the partial differential equations. The input variables of the computer program included fluidization velocity, steam flow rate and biomass to steam ratio. The model is capable of predicting the bed temperature, gas mole fractions, higher heating value and production rate.