Abstract: Electrorheological fluids (ER fluids) are typical non-aqueous suspension consisting of dielectric particles dispersed in non-conducting fluids, which show a reversible viscosity increase upon application of an external electric field. According to the obvious changes of the transmittance of ER fluids under an applied electric field, an assumption on the application of ER fluids to smart windows is presented in this paper. Applying and removing an electric field of 500 V.mm-1, a reversible solar transmittance change as much as 42.0% at the wavelength of 500 nm is experimentally obtained.