Pnly-N-metbylaniline/montmorillonite nanocomposite(PNMA-MMT) particles with high dielectric constant and suitable conductivity were prepared hy an intercalating polymerization method and characterized by IR

XRD and TEM

respectively．The structure analysis showedthatthelayer spacing ofMMT was swelledfrom 0.96 nm to 1.34 nm and the diameter of PNMA-MMT particles was about 30～100nm．The electrorheological(ER) properties of the suspensions of PNMA-MMT particles in silicone oil (20 wt％) were investigated under DC electric fields．It was found that the mechanic performance of PNMA-MMT ER fluids displays a notable ER effect under DC current field．The shear stress of PNMA-MMT ER fluid was 6.0 kPa(E=3 kV／mm

74.5 s-1)

which was 3.6 times as hish as that of the ER fluid at zero field．Furthermore

the sedimentation ratio of PNMA-MMT ER fluid was about 97％ after 60 days．The dielectric constant and the dielectric loss tangent of PNMA-MMT nanocomposites were higher than those of the two components．PNMA and MMT．It is apparent that the notable ER effect of PNMA-MMT ER fluid was attributed to the prominent dielectric property of the poly-N-methylaniline-montmorillonite nanocomposite particles．