Cotton fiber belonging to natural fibers at present has being used widely as substantial reinforced materials in polymer composite (PC). Polymer composites reinforced by cotton fiber have a lot of advantages such as low cost, high specific strength, light weight and biodegradability. Due to cotton fiber being hydrophilic, interfacial bond between the reinforcing fibers and the resin matrix is not firmly. To improve the interfacial bond between the reinforcing fibers and the resin matrix, cotton fiber has been treated with acetic anhydride. The acetylated cotton fiber becomes hydrophobic and has good bonding between the fibers and the resin matrix, Fourier Transform Infrared (FTIR) spectra indicates that cotton fibers are absorbed at the peak of 1740-1745 cm-1 and 1235 cm-1, it is clearly that hydroxyl groups were replaced by acetyl groups. Level of acetylation depends on reaction time and temperature, and it attains the highest effective value at reaction temperature of 80°C and reaction time of 9 hours. Tensile strength of polymer composite based on phenol formandehyde (PF) reinforced by acetylated woven fabric increases greatly at weight percent gain of 5.87 percent and then raises slowly.