Polyurethane (PU) films were synthesized from toluene 2,4-diisocyanate (TDI) and polyol. After introducing peroxides on the PU films through oxygen plasma treatment, PAA-grafted PU films were synthesized through the solution reaction of acrylic acid monomer with peroxides which are used as initiators. The maximum concentration of peroxides obtained by 1,1-diphenyl-2-picrylhydrazyl (DPPH) method was 2.0 nmol/cm(2) and we investigated the effect of reaction temperature on the change of grafting degree. The surface properties of the surface-modified PU films were characterized by the Attenuated Total Reflection-Fourier Transformed Infrared (ATR-FTIR) and Electron Spectroscopy for Chemical Analysis (ESCA). We measured the quantity of introduced -COOH groups using back-titration method. As results of this study, the amount of PAA grafted on PU film was increased with increasing reaction temperature and the apparent activation energy of this reaction was 39.5 kJ/mol. After comparing this result with other previous results, we could conclude that the activation energy of this reaction strongly depended on the type and the structure of substrate materials. Finally, since the dried PAA-grafted PU film showed locally-agglomerated non-uniform structure as the result of observations through ESCA and SEM, the PAA chain grafted on PU film certainly had locally different lengths due to the complex reaction paths.