Sulfonated polystyrene (PS) particles were prepared by the sulfonation of PS microspheres with H2SO4. Then, composite particles were synthesized by layer-by-layer (LbL) self-assembly with funtionalized multiwall carbon nanotubes (fMCNTs) and polyelectrolytes on sulfonated PS particles. The amount of fMCNTs on PS particles was adjusted by controlling the number of fMCNT layers by LbL self-assembly. Composite particles were characterized by zeta-potential analysis, scanning electron microscopy, and thermal analysis. The electrorheological (ER) properties of composite particles in insulating oil was investigated with varying the number of fMCNT layers under controlled electric fields. It was observed that the number of fMCNT layers was a critical factor to determine the ER properties of composite particles. (c) 2007 Wiley Periodicals, Inc.