In the present study the catalytic steam gasification of biomass to produce hydrogen-rich gas with calcined dolomite as catalyst in an externally heated fixed bed reactor was investigated. The influence of the reactor temperature and particle size on yield and product composition was studied at the temperature range of 700°C-900°C. Over the ranges of experimental conditions examined, tar was completely decomposed as temperature increases from 800°C to 900°C. Higher temperature and smaller particle size resulted in more H2 and CO2 production, and dry gas yield. The highest H2 content of 58.27 V%, and the highest H2 yield of 2.23 Nm3/ kg biomass were observed at the highest temperature level of 900°C when the particle size was below 0.125mm.