Despite the importance of cotton fabrics and its blends in our lives, the lack of their thermal stability is one of the most important point to be studied. The ability of hyperbranched poly urethane-urea (HBPU) to improve the thermal stability of cotton and cotton polyester (C/PET) fabrics was investigated. Different concentrations of HBPU were used for treatment of both cotton and C/PET fabrics. No significant effect on tensile strength properties was observed with cotton fabrics, while tensile strength of treated C/PET fabrics was affected by treatment with different concentrations of HBPU. TGA analysis shows enhanced char residue values with both treated cotton and C/PET fabrics. Polymer deposition on the surface of treated fabrics and structure changes were followed by SEM and FT-IR spectroscopy, respectively.