Biomass can be efficiently transformed into valuable gas products via gasification. Gasification converts biomass through partial oxidation into a gaseous mixture, small quantities of char and condensable compounds. In this work, an updraft gasifier is regarded because the updraft gasifier is suitable for wet fuels such as agricultural residues. Moreover, it is the most simple and can be used as a basis for the design and operation of small-scale gasifier. However, due to the high tar content of the gas produced, updraft gasifiers may not effectively be used without comprehensive gas cleaning. To improve an insight on the gasification process, app opriate mathematical models are required. Further, models are useful for gasifier design and operation such as prediction of operation behavior during normal operation, startup, and shutdown. This article presents a one-dimensional mathematical model for the simulation of a small-scale fixed-bed updraft gasifier of rice straw. The model is based on a set of differential eq ations describing the entire gasification process. The governing equations include conservation of mass and energy, complemented by boundary conditions, constitutive relationships. The main features of the model include homogeneous and heterogeneous combustion and gasification reactions, pyrolysis kinetics and drying, heat and mass transfer in the solid and gas phases as well as between phases. The simulation results were compared with experimental data, and showed good agreement. The model constructed was then used to predict the effects of varying moisture content and air feed temperature. Fuel with higher moisture content of fuel feed lowered the performance of the gasification process. But, higher temperature of air feed resulted in higher performance with lower tar content and more environmentally preferable.