We report very large enhancement of static yield stress for electrorheological fluids by adding ferroelectric nanoparticles of lead zirconate titanate (PZT) or lead titanate (PbTiO3) to ER fluids consisting of 50 μm glass spheres. It is found that the enhancement peaks at certain nanoparticle/microparticle ratios for fixed solid/liquid volume fractions. The results are explained by calculations using an effective medium approach, based on the physical picture that the nanoparticles modify the properties of the liquid and solid components.