Polyaniline (PANI) nanotubes were fabricated successfully using a micelle soft-template method in the presence of oxalic acid as a dopant and applied as the dispersed phase of an electrorheological (ER) fluid. The obtained nanotubes were on average 110 nm in external diameter and 10 nm in internal diameter, and several micrometers in length. The X-ray diffraction pattern confirmed the crystalline structure of PANI owing to its tubular morphology. The rheological characteristics of the flow curves and yield stress of its ER fluid when dispersed in silicone oil were examined using a rotational rheometer equipped with a high voltage generator. Under an electric field, the PANI nanotubes showed improved ER effects, confirming that elongated particles have enhanced ER properties due to the higher aspect ratio. The dielectric spectra also showed the relaxation time and polarizability of the PANI nanotube-based ER fluid, corresponding to its ER performance.