In the first project, poly(vinyl chloride)/silica nanocomposites were prepared. For this aim, firstly, the surface modification of silica nanoparticles was performed using citric acid and L-ascorbic acid as the bio-safe modifiers for the first time. The modified nanoparticles were identified using FT-IR, XRD, FE-SEM, TEM and TGA. The TGA results confirms the presence of about 13 % of modifiers on the silica surface. Then 4, 8, and 12 wt% of the modified nanoparticles were added to the poly(vinyl chloride) matrix and poly(vinyl chloride)/silica nanocomposite films were prepared under ultrasonic irradiation as a fast, economical and environmentally friendly method. The resulting nanocomposites were characterized using several techniques including FT-IR, XRD, FE-SEM, TEM, TGA, UV-Vis spectroscopy, and mechanical testing. The normal distribution curve showed the nanometer scale dispersion of nanoparticles in the polymer matrix. Also the mean size of nanoparticles was estimated 20 nm. The TGA results showed increased thermal stability for the nanocomposite films than the pure polymer. According to the UV-Vis spectrum, the nanocomposite films showed more absorption incomparison to the neat polymer. In the second project, poly(vinyl alcohol) was used as the polymer matrix and poly(vinyl alcohol)/silica nanocomposite films were prepared by the addition of 4, 8, and 12 wt% of modified silica to the poly(vinyl alcohol) matrix under ultrasonic irradiation. The resulting nanocomposites were characterized using different techniques including FT-IR, XRD, FESEM, TEM, TGA, UV-Vis spectroscopy, and mechanical test. The TEM micrographs of nanocomposite 4 wt% showed proper and uniform dispersion of the nanoparticles in the polymer matrix. According to the TGA thermograms, the addition of the nanoparticles to the polymer matrix leads to an increase in thermal resistance. Also, the nanocomposites showed more UV-Vis absorption than pure polymer.