The electrorheological (ER) response and dielectric properties of the polypyrrole (PPy) dispersions in mineral oil were investigated. Various PPy particles were synthesized controlling the amount of oxidant and surfactant to investigate the effect of these variables on the ER response. The ER response increases with the oxidant and surfactant amount, passes through a maximum, and then decreases with the oxidant and surfactant amount. However, the decrease in the ER response at large surfactant amounts is not so significant compared to that at large oxidant amounts. The increase in the ER response with the oxidant and surfactant amount arises from the enhanced particle polarization originated from the increased PPy conductivity. The decrease in the ER response at large oxidant amounts seems to arise from the increased conduction between the PPy particles, while that at large surfactant amounts arises from the decreased particle polarization.