Biomass from waste newspaper (WP) was first co-pyrolyzed with high-density polyethylene (HDPE) in order to enhance the oil yield and its fuel properties. The synergistic effects during co-pyrolysis were investigated in terms of entire pyrolysis process, products yield and properties of liquid products (aqueous phase and oil phase) using thermogravimetric analysis coupled with infrared spectroscopy (TG-FTIR), physical properties analysis, elemental analysis, Fourier transform infrared spectroscopy (FT-IR), and gas chromatography/mass spectrometry (GC/MS). The results showed that synergistic effect occurred at 400-500 degrees C resulting in an obviously increase in oil phase by 31.59% as compared to theoretical data. Positive synergistic effects on fuel properties of co-pyrolysis oil were observed, especially demonstrating dramatically decrease in viscosity and total acid number by 75.96% and 216.04% in comparison to theoretical data. WP pyrolyzed alone gives mainly oxygenated compounds in its derived oil, while HDPE give hydrocarbons. No cross reaction products appeared in co-pyrolysis oil, implying that the synergistic effects were determined by altering its compounds content rather than generating cross reaction products. Unfortunately, aqueous phase and oil phase exhibit similar composition. (C) 2016 Elsevier Ltd. All rights reserved.