A novel polyaniline nanofiber/kaolinite nanoplatelet hybrid nanocomposite was synthesized by means of rapidly mixed in situ polymerization. The resultant polyaniline/kaolinite hybrid nanocomposite was characterized via different techniques, such as X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results show that 2D clay nanoplatelets are coated by the 1D polyaniline nanofibers. The nanoclay platelets can improve the thermal stability of polyaniline nanofibers. An electrorheological effect is found with the suspension of polyaniline nanofiber/kaolinite nanoplatelet hybrid nanocomposite dispersed in silicone oil. (C) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1104-1113, 2013