The phosphoric acid doped conductive polyaniline (PANI) polymer nanocomposites(PNCs) filled with silica nanoparticles (NPs) have been successfully synthesizedusing a facile surface initiated polymerization method. The chemical structuresof the nanocomposites are characterized by Fourier transform infrared (FT-IR)spectroscopy. The enhanced thermal stability of the PNCs compared with that ofpure PANI is observed by thermogravimetric analysis (TGA). The dielectricproperties of these nanocomposites are strongly related to the silicananoparticle loading levels. Temperature dependent resistivity analysis revealsa quasi 3-dimensional variable range hopping (VRH) electrical conductionmechanism for the synthesized nanocomposite samples. A positive giantmagnetoresistance (GMR) is observed with a maximum value of 95.5% in the PNCswith a silica loading of 20.0 wt % and 65.6% for the pure PANI doped withphosphoric acid. The observed MR is well explained by wave function shrinkagemodel by calculating the changed localization length (ξ), density of states atthe Fermi level (N(EF)), and reduced average hoppinglength (Rhop). The effects of particle size on the propertiesincluding thermal stability, dielectric properties, temperature dependentresistivity, electrical conduction mechanism, and GMR of the nanocomposites arealso studied.