Co-liquefaction of microalgae (Spirulina) and synthetic polymer (HOPE, high-density polyethylene) in sub- and supercritical ethanol was investigated in a stainless steel autoclave (1000 mL) at different reaction temperatures (T), Spirulina/HDPE ratio (R-1), (Spirulina + HDPE)/ethanol ratio (R-2) and solvent filling ratio (R-3). Results showed that the addition of Spirulina to HOPE liquefaction could make the conversion conditions of HOPE milder. The yield of bio-oil obtained at 613 K with a 1/10 R-2 and a 2/10 R-3 was increased by 44.81 wt.% when the R-1 was raised from 0/10 to 4/6. Meanwhile, the synergetic effects (SE) between HDPE and Spirulina were increased from 0 to 30.39 wt.%. Further increasing R-1 resulted in a decrease in SE. The yields of bio-oil increased with increasing R-2 firstly and then declined. An opposite trend was observed for the yield of residue. The effect of R-3 to the yields of liquefaction products was similar to that of R-2. The content of C and H in bio-oils reduced with increasing RI, while the content of 0 increased. The bio-oil from pure Spirulina liquefaction runs mainly consisted of oxygen-containing compounds, such as carboxylic acids, esters and ketones. But the major components of bio-oil from co-liquefaction of Spirulina and HOPE mixture were similar to those of pure HDPE-derived bio-oil, in which aliphatic hydrocarbons dominated. (C) 2011 Elsevier B.V. All rights reserved.