A thermotropic liquid crystalline (LC) polymer, consisting of an LC silicone having benzoic acid phenylester LC groups as side chains of the siloxane polymer main chain diluted with dimethylsilicone, and a lyotropic LC polymer solution, consisting of poly(gamma-benzyl-L-glutamate) in 1,4-dioxane, both showed a large electrorheological (ER) effect, i.e., an instantaneous increase in shear stress upon the application of an electric field. In the electric field, the thermotropic polymer exhibited Newtonian-like flow and a dynamic viscoelasticity similar to that of low molecular weight liquid crystals, while the lyotropic polymer solution exhibited elastic flow and a dynamic viscoelasticity similar to that of particle-dispersion ER fluids. These differences in ER behavior suggest large differences in their ER mechanisms, with that of the thermotropic polymer dominated by interaction between its LC domains and that of the lyotropic polymer solution by the orientation of the dipoles of its LC groups.