The electrorheological properties of suspensions consisting of inorganic shell/organic core composite particles dispersed in a silicone oil were studied by the use of a stress-controlled rheometer. On application of electric fields, the suspensions show a striking increase in steady-shear viscosity. Since the constant stress is generated within the limit of zero shear rate, the plateau in the flow curve corresponds to the Bingham yield stress. When the stress does not exceed the yield value, the suspensions are expected to behave as elastic solids, However, even at stresses less than the Bingham yield stress, the suspensions show viscoelastic relaxation or plastic response with an infinite relaxation time. The development of yield stress in steady shear (dynamic yield stress) can be derived from the single-chain model in which the particles all align into fully developed chains of single-particle width and equal spacing. However, the ideal model cannot explain the appearance of static yield stress as a critical point, below which no flow can be observed and above which the substance is a liquid with a plastic viscosity. The creep experiments indicate that the static yield stress may be controlled by the chain structure at strains of about 0.4. The transition of column structure from the body-centered tetragonal lattice to another metastable configuration is reasonably consistent with the experimental findings.