H₂SO₄-modified TiO₂ particles were prepared using a sol-gel method followed by impregnating the dry gels with sulphuric acid and were used to fabricate an electrorheological (ER) fluid with high ER activity. The as-synthesized H₂SO₄-modified TiO₂ particles were characterized by fourier transform infrared (FT-IR), X-ray diffraction (XRD), BET N₂ adsorption-desorption isotherm analysis, and the ER effect and dielectric spectra of the ER fluids were measured. The results show that H₂SO₄-modified TiO₂ particles have smaller size of crystal grains and higher specific surface area due to the restraint of SO₄^2- from the grain growth. Under the same measured condition and electric field strength of 3kV/mm, the ratio of fieldinduced shear stress to zero field shear stress (τ_E/τ₀) of TiO₂ particles is 80, while that of H₂SO₄-modified TiO₂ particles is more than 500. The observed ER performances are reasonably explained by the stronger interfacial polarization, which is brought by the specific structures of the H₂SO₄-modified TiO₂ particles.