Surface-conductive microspheres consisting of poly(methyl methacrylate) (PMMA) (6.5 μm) core and carbon nanotubes (CNTs)-adsorbed shell were prepared using a simple process involving the blending of two colloidal solution; an aqueous CNT dispersion with surfactants and an aqueous PMMA microsphere colloid. These were adopted as the suspended particles for electrorheological (ER) fluids, in which the electrical conductivity originated primarily from the surface-coated conducting CNT layers. The CNT-adsorbed polymeric microspheres were monodisperse and spherical in shape. The CNT-PMMA composite suspensions in silicone oil showed the typical ER characteristics of forming a chain-like structure under an applied electric field. The CNT-PMMA composite microspheres exhibited a conductivity ranging from 5.2×10-4 to 6.3×10-5 S/cm, which is an acceptable conductivity range for ER fluids. This phenomenon can be explained by the interfacial polarizability of CNTs adsorbed on the surface of the polymeric microspheres.