The main objective of this paper is to study the effect of design and operating parameters, mainly reactor geometry, equivalence ratio and biomass feeding rate, on the performance of the gasification process of biomass in a three air stage continuous fixed bed downdraft reactor. The gasification of corn straw was carried out in the gasifier under atmospheric pressure, using air as gasifying agent. The results demonstrated that due to the three stage of air supply, a high and uniform temperature was achieved in the oxidation and reduction zones for better tar cracking. The designing of both the air supply system and rotating grate avoided bridging and channeling. The gas composition and tar yield were affected by the parameters including equivalence ratio (ER) and biomass feeding rate. When biomass feeding rate was 7.5 kg/h and ER was 0.25-0.27, the product gas of the gasifier attained a good condition with lower heating value (LHV) about 5400 kJ/m(3) and cold gas efficiency about 65%. An increase in equivalence ratio led to higher temperature which in turn resulted in lower tar yield which was only 0.52 g/Nm(3) at ER = 0.32. Increasing biomass feeding rate led to higher biomass consumption rate and process temperature. However, excessively high feeding rate was unbeneficial for biomass gasification cracking and reforming reactions, which led to a decrease in H-2 and CO concentrations and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the lower heating value decreased from 5455.5 kJ/Nm(3) to 5253.2 kJ/Nm(3) and tar yield increased from 0.82 g/Nm(3) to 2.78 g/Nm(3). Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved