In this thesis, firstly, the surface modification of the alumina nanoparticles was performed in order to prevent aggregation between hydrophilic surface of the nanoparticles and improvement of the nanoparticles compatibility with the polymer matrix. The modification was performed using citric acid (CA) and ascorbic acid (AA) in the presence of ultrasonic irradiation. The modified alumina nanoparticles were characterized using FT-IR, XRD, TGA, FE-SEM, and TEM. The TGA results indicate the presence of citric acid and ascorbic acid on the surface of the alumina nanoparticles in comparison with the pure nanoparticles. Based on the XRD patterns, the surface modification had no effect on the crystalline structure of the alumina nanoparticles. After surface modification of the nanoparticles, poly(vinyl chloride) (PVC) was selected as a polymer matrix due to its good structural characteristics. Then, the PVC/?-Al 2 O 3 -CA-AA nanocomposites containing 4, 8 and 12 wt % of modified nanoparticles were synthesized under ultrasonic irradiation. The study of the nanocomposites morphology with FE-SEM and TEM indicates a homogenous dispersion of the nanoparticles in the polymer matrix. In the second study, poly(vinyl alcohol)(PVA) was chosen as another polymer matrix. PVA contains lots of hydroxyl groups which can react with the hydroxyl and carboxyl functional groups of citric acid and ascorbic acid, even with the hydroxyl functional groups on the surface of the alumina nanoparticles. In addition, PVA is nontoxic, biodegradable, and water soluble. The FE-SEM and TEM results show a homogenous dispersion of the modified nanoparticles into the polymer matrix.