Polyurethane (PU) films were prepared by solution-casting through co-polymerization between liquefied wood (LW) and polymeric methylene diphenylene diisocyanate (PMDI) at [NCO]/[OH] ratios of 1.0 and 1.2. The LWs tested were made from six wood species through liquefaction using glycerol-polyethylene glycol (PEG) co-solvent, in the presence of sulfuric acid at 150degreesC. The viscosity of the LWs at 25degreesC varied from 1.37 to 2.31 Pa s with the wood species, whereas the hydroxyl number, moisture content, and amount of dissolved woody components (DWC) were almost constant. The PU films prepared from LW with high viscosity were found to be more rigid than the films prepared from LW with low viscosity. The increase in the viscosity contributed to the increases in the crosslink density of the PU films. Varying the viscosity is a way to control the mechanical properties of PU films at a constant [NCO]/[OH] ratio.