||Electrorheological (ER) Fluids, consisting of dielectric particles dispersed in insulating fluids, are a class of materials whose rheological properties are controllable by the application of an electric field. This unique property has great potential applications in various fields. In this thesis, I report measurements of shear modulus and static yield stress of two-phase ER-fluids obtained by mixing nano-sized Lead Zirconate Titanate (PZT) particles with micron-sized (~55μm) glass spheres. It is found that the shear modulus of the two-phase ER-fluids, overall, decreases with increasing frequency of the applied electric field. More importantly, soft-mode-like dips as predicted by first-principle calculations are observed for some particle ratios and volume fractions. In contrast to the mono-dispersed glass ER-fluids the static yield stress of the two-phase ER-fluid is found to increase with increasing frequency of the applied electric field. Furthermore, the structures of the ER-fluids observed by optical microscope are found to be frequency dependent.