Dielectric behaviors of titanium dioxide (TiO2)-based electrorheological (ER) suspensions with different particle concentrations and TiO2 polymorphs were investigated in the frequency range of 40 Hz to 110 MHz. Two relaxations in kilohertz and megahertz frequency range were attributed to interface polarization between TiO2 and silicone oil and ion pair polarization between dissociated counterions and fixed charges on TiO2 surfaces, respectively. Dipolar coefficient D, which is related to the construction or structure of the colloid, changes after critical volume fraction 0.05, indicating that chain-like or network structures are formed by particles. Based on percolation model, the values of critical exponent suggest that particles may form two-dimensional percolation network. Furthermore, the effective dielectric mismatch parameter, , was calculated based on the obtained phase parameters. We found that rutile should have better ER activity than anatase. The main reason for weak ER activity of pure TiO2 ER suspensions may due to poor conductivity properties of TiO2 crystals.