HTC (hydrothermal carbonization) is a technically-attractive thermal conversion process for biomass to produce solid carbonaceous products at mild conditions. EB (eucalyptus bark) was used as a feedstock for producing hydrochar by HTC. Effect of process conditions on the yield and physicochemical properties of hydrochar was examined by varying carbonization temperature over the range of 220–300 °C and varying residence time over the range of 2–10 h. With increase in temperature, the hydrochar yield decreased slightly from 46.4% at 220 °C to 40.0% at 300 °C. The O/C and H/C atomic ratios reduced from 1.69 and 0.80 to 0.83 and 0.23, respectively, which was mostly related to dehydration, decarboxylation and demethanation reactions. The oxygen containing functional groups decreased with increasing temperature. HHV (higher heating value) of hydrochar was in the range of 20.2–29.2 MJ/kg. Thermogravimetric analysis showed that hydrochar products obtained at temperature over 220 °C exhibited almost the same thermal behaviors. In comparison, the influence of residence time on the yield, physicochemical properties and thermal behavior of hydrochar was marginal. •Hydrothermal carbonization was employed to convert eucalyptus bark into hydrochar.•Carbonization temperature had a significant effect on the formation of hydrochar.•Residence time had marginal influence on the hydrothermal reactions.•Energetic properties of hydrochar was improved by hydrothermal carbonization.•Primary mechanisms of hydrochar formation were proposed.