A composite material of a silica-based mesoporous molecular sieve, MCM-41, with conducting polyaniline (PANI) inside the uniformly aligned one-dimensional channels (PANI/MCM-41) was prepared and its nanocomposite formation was confirmed through an electrical conductivity measurement. This nanocomposite particle was adopted for a dispersed phase in electrorheological (ER) fluids, and the ER property was measured using a Couette-type rotational rheometer equipped with a high voltage generator. Suspension of PANI/MCM-41 showed ER properties more enhanced than those of MCM-41 or PANI alone as a result of the anisotropic polarization of the PANI/MCM-41 nanocomposite.