The purpose of this study is to investigate the effects of the biomass particle size on the changes in char structure during the gasification in steam of mallee woody biomass. The experiments with continuous or sudden supply of steam were conducted using a fluidized-bed/fixed-bed quartz reactor. The structural features of chars were then characterized using Fourier transform (FT)-Raman spectroscopy. Our results indicate that the effects of the biomass particle size on char structure were minimal when the biomass was heated slowly while being in continuous contact with steam. On the contrary, when the biomass was first pyrolyzed in argon and then suddenly came into contact with steam, the biomass particle size could have a profound effect on the subsequent evolution of char structure during gasification. For the large biomass particles (>5 mm), the ratio between small and big aromatic ring systems in the char showed a maximum value after around 30 s of contact with steam.