Two types of amorphous TiO2 particles with different particle sizes were synthesized by a simple sol-gel method and were characterized by X-ray diffraction analysis, field emission scanning electron microscopy, and Fourier transform infrared spectrometry. The electrorheological (ER) results show that the TiO2/silicone oil suspensions exhibited a remarkable ER effect. The static shear stress can be up to 130 kPa (shear rate 0.2 s (-aEuro parts per thousand 1)) under the DC electric field of 4 kV/mm at room temperature. The polar molecules present on the particles' surface play a decisive role for the observed giant ER effect, which arises from the alignment of polar molecules in the gap between neighboring particles.