This study was to synthesize the inherently conductive polymer polyaniline using an optimized process to prepare polyaniline/silicon dioxide (PANI/SiO2) composites by in situ polymerization and ex situ solution mixing. PANI and PANI/SiO2 composite films were prepared by drop-by-drop and spin-coating methods. The morphology of particles and films were examined by a scanning electron microscope (SEM). SEM measurements indicated that the SiO2 were well-dispersed and isolated in composite films. The electrorheological (ER), characteristics of the PANI/SiO2 composites were investigated. A volume fraction series (phi = 5-25 %) of the PANI/SiO2/silicone oil dispersions were prepared and sedimentation stabilities were determined. An ER activity was observed from the samples, when subjected to external electric field strength thus, they were classified as smart materials. Some parameters affecting the ER properties of the dispersions such as volume fraction, shear rate, electric field strength, frequency, and temperature were investigated.