Abstract: In the present study, a stoichiometric thermodynamic equilibrium model has been introduced for downdraft biomass air gasifiers. Experimental data from five pilot scale downdraft gasifiers, that use different types of biomass fuels, are compared with the findings of the model. It is presented that the developed model is efficiently simulating H2, CO, CH4, and CO2, volume fractions and the heating value of synthesis gas. The minimum error of comparisons has been found about 1 and the maximum error was obtained to be less than 25. The level of sensitivity evaluation has been performed to the model to check out the effect of gasifier temperature, biomass moisture content, air to fuel ratio and equivalence ratio on the syngas form and lower heating value of syngas from the downdraft gasifier. The model results are compared with several existing experimental data of the literature. It is concluded that the findings of the study are a good candidate to model the downdraft gasifiers. © 2019, Allerton Press, Inc.