A hydrothermal treatment experiment of eucalyptus wood was carried out in a high-pressure batch reactor at temperatures between 160 and 190 degrees C. The effect on chemical structure and pyrolysis behavior of eucalyptus wood as a result of hydrothermal treatment was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric (TG) analysis, and pyrolysis gas chromatography mass spectroscopy (Py-GC/MS). From FTIR analysis, it was observed that hydrothermal treatment could effectively remove acetyl groups from eucalyptus wood. XRD analysis showed that crystallinity degree of eucalyptus wood was enhanced by hydrothermal treatment due to the degradation of hemicellulose and amorphous cellulose, and crystalline size of cellulose became larger owing to the removal of small crystallites. TG analysis suggested that no significant carbonization and cross-linking of chemical components in eucalyptus wood occurred during hydrothermal treatment, and the thermal stability of eucalyptus wood was enhanced. Compared to raw eucalyptus wood, hydrothermally treated eucalyptus wood gave much higher levoglucosan yields but lower yields of reactive compounds including ketones, aldehydes, and organic acids in the Py-GC/MS experiment. This implied that hydrothermal treatment had positive impacts on biomass pyrolysis product distribution and could improve chemical composition of bio-oil produced from fast pyrolysis.