Highly monodisperse silica nanoparticles with four different diameters (100, 150, 250 and 500 nm) are prepared by adjusting the reaction temperature. With increasing the reaction temperature, the nucleation is promoted, consequently reducing the particle size. As-prepared silica particles are adopted as dispersing materials for electro-responsive smart fluids. The influence of particle size on the electroresponsive characteristics is systematically studied and analyzed. The ER efficiency of the silica-based ER fluids is significantly enhanced from ca. 17 to 63 as the particle size increases from 100 to 500 nm at a shear rate of 1 s-1 under the electric field of 2.5 kV mm-1. Furthermore, the anti-sedimentation properties of the silica nanoparticles are investigated to achieve an in-depth insight into the effect of particle size on electrorheological performances. All silica-based ER fluids show excellent anti-settling property that maintains high sedimentation ratio over 0.987 even after 120 h.