Polyaniline-coated mesoporous silica (PANI/mSiO(2)) materials with different aspect ratios (L/D = 1, 5, and 10) were fabricated by a vapor deposition polymerization (VDP) method to investigate the geometric effect on electrorheological (ER) activity. The PANI/mSiO(2) materials were dedoped by a facile NH4OH treatment to reduce the conductivity to a level appropriate for ER applications. Notably, the PANI/mSiO(2)-based ER fluids exhibited enhanced ER performance with increasing aspect ratio. In particular, the PANI/mSiO(2) material with the highest aspect ratio manifested the highest ER activity, which was attributed to geometric effects on flow resistance and mechanical stability. Moreover, the ER materials with higher aspect ratios showed improved dielectric properties of large achievable polarizability and short relaxation time. Hence, the synergistic contribution of geometric effects and dielectric properties resulted in enhanced ER activity. Consequently, this study provides insight into an effective method to improve ER performance by simple manipulation of the particle geometry. (C) 2016 Published by Elsevier Inc.