The effects of two kinds of wastewater (black liquor and paper regeneration wastewater) from paper industry as solvent on the hydrothermal liquefaction of plantation biomass were discussed in this study. For the solvent of eucalyptus, black liquor was not suitable because the oil yield was very low and the yield of water-soluble constituent was very high. There was little difference in the product yields between the liquefaction of eucalyptus with the paper regeneration wastewater (pH 9.5) and that with NaOH aqueous solution (pH 9.5). In comparison with the liquefaction with water, the decrease of residue yield and the increase of oil yield were observed on the liquefaction with the wastewater. During the liquefaction with water, it was anticipated that dehydration and polymerization between carboxylic and phenolic hydroxyl groups in the molecules of the eucalyptus occurred. On the other hand, carboxylic acid formed during the liquefaction of eucalyptus was considered to be neutralized with Na+ in NaOH aqueous solution or cations in the wastewater because the formation of carboxylate was observed in the water-soluble constituents derived from these liquefactions. Therefore, the residue yield decreased owing to the inhibition of the condensation reaction. Accordingly, possibility for the reduction of the amount of greenhouse gas (CO2) and paper regeneration wastewater, and the production of oil from eucalyptus was clarified on the liquefaction of plantation biomass (eucalyptus) with paper regeneration wastewater.