Abstract: In electrorheological (ER) fluids the shear strength or hardness can be continuously and rapidly tuned by applying an electric field, thus these media promise significant applications in a wide range of industries. Ordinary ER fluids operate on the basis of mutual interaction among the polarized particles, consequently the maximum yield stress available, according to the dielectric theory, is only about 10kPa. This low yield stress has deferred the implementation of ER effects for half a century since its discovery. In recent years we succeeded in making a variety of new ER fluids, known as polar molecule dominated ER fluids, in which the yield stress can be raised by an order of magnitude or even more. The yield stress shows a linear dependence on the electric field, which differs from the quadratic relation for ordinary ER fluids. We postulate that this extraordinarily large effect originates in the polar molecules aligned by the strong local field among particles, i.e., via the polar molecule-polarization charge interaction. This model can explain the experimental phenomena observed in polar molecule dominated ER fluids, and it also promises the possibility of obtaining a yield stress on the order of MPa through exploitation of this mechanism.