The catalytic steam gasification of biomass was carried out in a lab-scale fixed bed reactor in order to evaluate the effects of particle size at different bed temperatures on the gasification performance. The bed temperature was varied from 600 to 900 degrees C and the biomass was separated into five different size fractions (below 0.075 mm, 0.075-0.15 mm, 0.15-0.3 mm, 0.3-0.6 mm. and 0.6-1.2 mm). The results show that with decreasing particle size, the dry gas yield, carbon conversion efficiency and H(2) yield increased, and the content of char and tar decreased. And the differences due to particle sizes in gasification performance practically disappear as the higher temperature bound is approached. Hydrogen and carbon monoxide contents in the produced gas increase with decreasing particle size at 900 degrees C, reaching to 51.2% and 22.4%, respectively. Crown Copyright (C) 2008 Published by Elsevier Ltd on behalf of international Association for Hydrogen Energy. All rights reserved.