Abstract: Electrorheological (ER) fluids were fabricated by dispersing oxalate group-modified amorphous titanium oxide nanoparticles in dimethyl silicone oil with different kinematic viscosities (10 cSt, 50 cSt, 100 cSt, 500 cSt and 1000 cSt). The yield stress, zero-field viscosity, ER efficiency, response time, and sedimentation stability were tested to find the influence of the viscosity of silicone oil on the properties of ER fluids. The results indicate that the ER fluids based on the silicone oil with kinematic viscosity of 50 cSt present the optimal ER efficiency, and the fluids based on the silicone oil with kinematic viscosity of 100 cSt show the shortest response time and good sedimentation stability. The mechanism is that the silicone oil with larger viscosity provides larger viscous force, but makes the nanoparticles easier to agglomerate.