Abstract: Gasification processes are complicated with numerous parameters. It makes us difficult to understand the whole process nature. Therefore, a simplified approach based on modified stoichiometric equilibrium model for downdraft gasifier is designed in order to simulate the overall gasification process. The model uses the chemical reactions and species involved, while it starts by selecting all species containing C, H, O and N. Mass and energy balances of the gasifier -coupled with two equilibrium reactions are incorporated to create the model. This model can be solved with user friendly Micorsoft Excel and nonlinear Matlab solver. To validate the model, a close agreement is shown between numerical and experimental results within the same configuration. It can predict the composition of producer gases and heating values with acceptable accuracy. After validation, it is simulated to study the effects of temperature, moisture content of biomass and air-fuel ratio (related to equivalence ratio) in the downdraft gasifier. And then, experiments are conducted using optimized simulation data obtained from the model and it was found that the output producer gases were a close agreement with model results in producer gas composition and heating value.