Poplar liquefaction (PL) in methanol, water, or water/methanol cosolvents (VVMCSs) was investigated at 240-320 degrees C for 0-90 min. The results show that the yields of bio-oils (BOs) obtained from PL in WMCSs are higher than those in either methanol or water, indicating that methanol has a synergic effect with water on PL. The maximum BC) yield of 44.2% was obtained at 270 degrees C for 15 min in a WMCS containing 70 vol % water. The BOs were analyzed with a gas chromatograph/mass spectrometer (GC/MS) and Fourier transform infrared (FTIR) spectrometer. Poplar and its residues were analyzed with the FTIR spectrometer and a scanning electron microscope (SEM). The fiber structure of poplar was significantly destroyed during PL in WMCS based on the SEM observation. According to GC/MS analysis, the BOs mainly consist of hydrocarbons, phenols, furans, other ethers, aldehydes, ketones, carboxylic acids, esters, and nitrogen-containing organic compounds. Among of them, phenols, ketones, and esters are the main group components. To investigate the liquefaction mechanism, the three major components in biomass, i.e., cellulose, hemicellulose, and lignin, were subjected to degradation in the same solvents. The results suggest that WMCS exhibits better synergic effects for cellulose and hemicellulose than for lignin. Further investigations are needed for a detailed mechanism on synergic effects.