An Australian mallee wood of different particle sizes was gasified in steam in a fluidized-bed reactor at different temperatures. The structural features of chars, combustion reactivity of chars, and alkali and alkaline earth metallic (AAEM) species retention in chars were investigated in view to elucidate how the particle size affects the carbon conversion during steam gasification of biomass. The structural features and combustion reactivity of chars were investigated using Raman spectroscopy and thermogravimetric anlysis, respectively. The Raman intensity and combustion reactivity of chars were seen to decrease with increasing temperature. However, the combustion reactivity of char increased with increasing biomass particle size in the bigger particle range (1.5-5.18 mm). This is due mainly to the increase of catalytic species (AAEM) retention in chars. The increased AAEM retention and condensation of aromatic ring systems are a result of increasing mass transfer resistance in bigger particles of biomass.