The co-liquefaction of biomass and high-density polyethylene (HDPE) mixtures in sub- and supercritical water was studied in a 500 mL stainless steel autoclave. The effects of reaction temperature (613-713 K), holding time (0-80 min), water to biomass-HDPE ratio (4-10), water filling ratio (6-18 vol.%) and biomass-plastic composition (100/0, 80/20, 50/50, 20/80 and 0/100) on the liquefaction of biomass and plastic to oils were investigated in 15.2-27.1 MPa. At 653 K, the most important parameter for the oil yield was biomass/HDPE ratio in the feedstock, and oil yield close to 60 wt.% was obtained for the 20/80 weight ratio of biomass/HDPE mixture. In some cases, non-additive effects were observed, leading to higher yield of oils. Results showed that the addition of biomass to HDPE liquefaction could make the reaction conditions milder, and enhance the conversion of HDPE at lower temperature, implying the synergistic effect of biomass and HDPE. The oils were analyzed by GC-MS and Elemental Analyzer. indicating that the oil from co-liquelaction was better in quality, comparing with that produced from pure biomass. (c) 2008 Elsevier B.V. All rights reserved.