We prepared polyaniline (PANI)/clay composites that are composed of both PANI-clay nanocomposite particles and pure PANI particles. The PANI-clay nanocomposite particles were made during the polymerization process, wherein PANI particles are attached on the surface of exfoliated clay particles modified by an aminosilane group. The PANI/clay composites were used as electrorheological fluids (ERFs) by dispersing them in silicon oil. The PANI-clay nanocomposite particles, which can form columnar structure under an electric field, strongly enhance the mechanical rigidity of the suspensions. The maximum yield stress of the PANI/clay composite suspensions (15 wt% in silicon oil) was 1.6 kPa at 3 kV/mm, while that of pure PANI was 300 Pa at the same electric field. A mechanism to explain the yield behaviors of the PANI-based nanocomposite suspensions is proposed.