It is proved experimentally and theoretically that the electrorheology effect (ER) dominantly depends on the electrical conductivity. The mechanism of electrorheology includes not only the body and the interface polarizations but also the ion polarization of double electric layers. Under direct and low-frequency alternate electric field, the ER fluid exhibiting out standing ER effect requires σp＞σf and εp＞εf. It is beneficial to improve rheological proper ties of ER fluids by increasing the σf and the static electric interaction of the ion layers. But it must work under a low electric current and low Joule heat and low electric power input.