A mathematical model of downdraft gasifier has been developed to simulate steady and transient state of gasification process. The model is based on lumped capacitance method and chemical equilibrium in pyrolysis and oxidation zone and one-dimensional reaction kinetics in reduction zone. Effects of the equivalence ratio (0.37-0.45), steam to biomass ratio (0-4) and mass flow rate of biomass (18-25 kg h(-1)) on the steady and transient characteristics of gasifier have been studied. The model gains good agreement with experimental results for exit gas composition. A steady analysis reveals that higher equivalence ratio (ER), lower steam to biomass ratio (S/B) result in a higher reaction temperature, which promotes the yield of H-2 and CO and the char conversion into syngas in this work. For S/B = 0 and mass flow rate of biomass equal to 20.9 kg h(-1), char conversion rate increases from 76% to 100% due to increasing of ER from 0.39 to 0.43. For ER = 0.43 and mass flow rate of biomass equal to 20.9kg h(-1), char conversion rate decreases from 84% to 24% due to increasing of S/B from 1 to 4. The molar fraction of gas composition almost remains stable for mass flow rate of biomass in the range of 18-22 kg h(-1).The large mass flow rate of biomass is obviously unfavorable for char conversion due to low temperature. For steam flow rate equal to 0.247 mol s(-1) and air flow rate equal to 0.5326 mol s(-1), the char conversion rate declines from 100% to 71% due to mass flow rate of biomass increasing from 19 to 22 kg h(-1).The higher flow rate of H2O relies on less yield of H-2 if ER is less or S/B is higher corresponding to more steam inducted into gasifier. Transient analysis shows that flow rate of H-2 goes down then up with increasing of ER due to competition between oxidation and reduction reactions. As S/B changing alone, the yield of H2O is absolutely dominated by inlet steam whether flow rate of H-2 increases or not.