Abstract: Poly-o-phenylenediamine/montmorillonite nanocomposite (PoPD-MMT) particles were synthesized by an emulsion intercalation method and characterized by IR, XRD and TEM respectively. The structure analysis shows that the diameter of PoPD-MMT particles is about 100 nm. The electrorheological properties of the suspensions of PoPD-MMT particles in silicone oil (Volume fraction,22%) have been investigated under DC electric fields. It was found that the mechanic performance of PoPD-MMT ER fluids displays a notable ER effect. The shear stress of PoPD-MMT ER fluid is 8.27 kPa (3 kV/mm, 74.5 s^-1), which is 2.5 times that of the electrorheological fluid at zero field, and much higher than that of pure poly-o-phenylenediamine (PoPD) and pure montmorillonite (MMT). Furthermore, the sedimentation ratio of PoPD-MMT ERF was about 97% after 60 days. The dielectric constant and the dielectric loss tangent of PoPD-MMT nanocomposite were higher than those of PoPD and MMT. It is apparent that the notable ER effect of PoPD-MMT ER fluid was attributed to the prominent dielectric property of the poly-o-phenylenediamine/montmorillonite nanocomposite particles.