Barium titanate (BaTiO3) inorganic particles which possess large electronic resistance and excellent dielectric properties were employed to synthesize conducting polyaniline (PANI)/BaTiO3 composites via an in situ oxidative polymerization, since conducting PANI/inorganic composites have been considered as a superior candidate of electrorheological (ER) fluids because of their physical properties, unique structure, and the combined merits of the two phases. The influence of the fraction of BaTiO3 particles in the as-synthesized composites on the physical properties (morphology and crystal structure) and the ER behaviors were examined. Yield stress data obtained were analyzed based on the universal yield stress equation as a function of applied electric field and it was found that the universal yield stress equation collapses these data onto a single curve independent of BaTiO3 particle concentration. Their shear stresses under an applied electric field were also found to be fitted well with the Cho-Choi-jhon model. In addition, the investigated dielectric spectra were found to be useful to interpret the differences in the ER performances for the PANI/BaTiO3 composite based ER fluids. (c) 2007 Wiley Periodicals, Inc.