In this work we use ground tire rubber (GTR) powder obtained by grinding worn tire treads as reinforcer agent in flexible polyurethane (PU). Characterization of the microstructure of the as-received powder is achieved using a series of standard techniques including scanning electron microscopy (SEM), granulometry-laser, Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction (XRD). To have complementary physical information the composition and thermal characteristics of the GTR powder, thermogravimetry analysis (TGA) is also performed. The set of these preliminary characterizations shows that the GTR powder particles can be used as reinforcing fillers. For the purpose of good compatibility with the PU matrix, the GTR powder is subjected to chemical treatments for reducing the impurities on the powder particles and to create functional groups at their surface. Subsequently, a series of GTR/PU composite samples are prepared with different weight fractions of GTR using free rising foam method. The GTR/PU composites are then characterized to assess the effect of the GTR content and their chemically pretreatment on thermal stability, compression mechanical behavior as well as sound attenuation properties. Collectively, these results indicate a significant improvement of both thermal and mechanical properties of the GTR/PU composites compared to the pristine PU matrix. Furthermore, it is also emphasized that the sound absorption response shows a significant shift of the maximum of the absorption coefficient toward lower frequencies resulting from simultaneous increase in air-flow resistivity and tortuosity which can have great potential application in the field of underwater acoustics. The effects of chemical treatments and GTR amount are also discussed. It is also shown that the results show improvement when H 2 O 2 solvent is used in comparison with NaOH, and the optimal properties are reached for PU samples containing 20 wt% of GTR whatever the pre-treatment is.