In this paper, composite electrorheological (ER) fluids were prepared by blending piezoelectric particles (PEPs) with organic polymer ER particles in silicone oil. The shear stress of the composites was greatly improved by adding surfactant polyethylene glycol (PEG) to disperse the particle aggregation. Composites with 500 nm BaTiO3 showed better ER properties than that of the 50 nm BaTiO3 composite. Nano-sized SiO2 powder was effective in making the composite suspension stable. Optic microscope observation indicated that the particles in the composite ER fluid formed a chain structure under an applied electric field. Compared with the DC electric field, the high-frequency AC electric field made the chain structure of fluids thinner and weakened the strength of the chain. The main reason was considered to be that the response time of the ER particles was of the microsecond order. (C) 2019 The Japan Society of Applied Physics