NOVELTY - Producing an upgraded bio-oil from a wet biomass using hydrothermal liquefaction, comprises: providing wet biomass comprising water (at least 70 wt.%) and biomass solids (less than 30 wt.%); heating wet biomass at first temperature (200-400 degrees C) under first pressure (0.1-25 Mpa) for 10-200 minutes to form crude bio-oil; and heating crude bio-oil and water at second temperature under second pressure for at least 30 minutes to form the upgraded bio-oil, where the second temperature is greater than the first temperature and is at least 300 degrees C.    USE - The method is useful for producing an upgraded bio-oil from a wet biomass.    ADVANTAGE - The method: utilizes aqueous phase processing and avoids the use of solvents, thus reducing costs, environmental impacts and energy requirements; and provides upgraded bio-oil having excellent heating value.    DETAILED DESCRIPTION - Producing an upgraded bio-oil from a wet biomass using hydrothermal liquefaction, comprises: providing the wet biomass comprising water (at least 70 wt.%) and biomass solids (less than 30 wt.%); heating the wet biomass at a first temperature (200-400 degrees C) under a first pressure (0.1-25 Mpa) for 10-200 minutes to form a crude bio-oil, where at least a portion of the water present in the wet biomass remains in a liquid phase throughout the heating step to form the crude bio-oil, and the first temperature and first pressure are below super-critical conditions of water; and heating the crude bio-oil and water at a second temperature and a second pressure for at least 30 minutes to form the upgraded bio-oil, where at least a portion of the water remains in the liquid phase or in a super-critical fluid phase throughout the heating step to form the upgraded bio-oil, and the second temperature is greater than the first temperature and is at least 300 degrees C. An INDEPENDENT CLAIM is also included for the upgraded bio-oil obtained from the refinement of crude bio-oil produced by the hydrothermal liquefaction of wet biomass, where the heating value of the upgraded bio-oil is 35-55 MJ/kg.