Tar formation is practically unavoidable during gasification. Catalytic tar forming is one of the most promising techniques for producing high-quality syngas at a commercial scale. Biochar has great potential to be used as a catalyst for the removal of tar from the syngas produced from the pyrolysis/gasification of biomass. The structure of biochar is a critical factor affecting its catalytic performance. This study investigates the role of O-containing functional groups in biochar during steam reforming of tar using biochar as a catalyst. Mallee wood biochar (106-250 mu m) was activated in H2O for different times (0-50 min) and then used as a catalyst for the steam reforming of tar at 800 degrees C. The chemical structural features of biochars were characterized with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that H2O activation increased the concentration of O-containing functional groups, mainly the aromatic C-O structures in biochar, which enhanced the catalytic destruction of tar. During the catalytic reforming of tar, the content of aromatic C-O groups decreased while the catalyst activity decreased. It is believed that the aromatic C-O functional groups in biochar play a vital role in the steam reforming of tar.