A four-parameter model (Seo-Seo model) was used to analyze the flow behavior of some electrorheological (ER) fluids containing polypyrrole (PPy) nanoparticles, nanocomposite particles of conductive polypyrrole confined in mesoporous silica (MCM-41), and core-shell-structured SiO2/polypyrrole nanoparticles. The static yield stress predictions by the model were compared with the experimental data and dynamic yield stress obtained from the Bingham model and/or Cho-Choi-Jhon (CCJ) model. The static yield stress values were larger than the dynamic yield stress values. It was also found that the static yield stress of the polypyrrole suspension had a quadratic dependence on the electric field strength as predicted by the electric polarization model whereas those of the nanocomposite suspensions showed 1.5 power-law dependency. A master curve describing the yield stress data dependence on the electric field strength was obtained using a single-parameter scaling function to interpret the underlying mechanism of ER activity. A simple method for evaluating the activity mechanism criterion has been proposed and applied to the ER response of those three kinds of suspension. The results show that the critical electric field strength should be checked before the conduction mechanism is asserted, even if the yield stress plot shows 1.5 power-law dependence on the electric field strength.