In this study, electrorheological (ER) properties of polyaniline (PANI) and polyaniline/red mud (RM) composites were investigated. Polyaniline and polyaniline/red mud (PRM) composites were prepared by oxidative polymerization. PAN I and PRM particle-based ER suspensions were prepared in silicone oil (SO) and their ER behavior was investigated as a function of concentration, shear rate, electric field strength, frequency, and temperature. Sedimentation stabilities of these suspensions were determined. ER activities of all the suspensions were observed to increase with increasing electric field strength, dispersed particle concentration, and decreasing shear rate. T, he suspensions show shear thinning non-Newtonian viscoelastic behavior, in which viscosity of suspension decreases with increasing shear rate. Shear stress of composite suspensions increases linearly with increasing applied electric field strength and concentration of the particles. Effect of high temperature on ER activity of ER fluids system was also investigated.