Biomass is a form of energy that has been increased substantially in the last decade. Gasification is a technology that converts solid biomass to fuel gas. It is a simple and economic process to generate power in rural areas. Gasification always use woodchip because it is a form of biomass which is produced from tree branches as well as agricultural residues. The pressure drop occurred in the downdraft gasifier. It is necessary to study the effect of woodchip particle size on pressure drop in the case of modeling gasification and gasifier structure design. The objective of this paper is to determine the effect of chip size distribution on air flow and woodchip height through fixed bed of wood chips. The woodchip was uesd 0.7~0.9mm and 20~3mm size. Air pressure drop experiments were conducted in a 2.5 meter high vertical transparent acrylic pipe. The result was presented that pressure drop increase with bed height and flow rate. Pressure drop occurred high when use 0.7~0.9mm size than 20~3mm size woodchip. Porosity is the major parameters on pressure drop.