We have studied the dielectric and rheological characteristics of semiconductive polymer based ER fluids. Under static electric field, yield stress of suspension was found to be closely related to the dielectric properties: that is in accord with the Maxwell-Wagner interfacial polarization. The dielectric constant showed maximum then decreased with increasing bulk conductivity of particle as the yield stress behaviors did. Particle-medium interaction on ER response was also examined by utilizing appropriate surfactant. These experimental results were consistent with the prediction based on the recent model studies. Over wide range of temperature, our ER fluid maintains its activity so that it can broaden the potential applicability as a smart material.