Changes of the rheology under the application of an electric field, electrorheological (ER) effect, are studied in the smectic A (SmA) and the nematic (Ne) phases of 4-n-octyl-4’-cyanobiphenyl (8CB). In the SmA phase, the electric field induces a large decrease in the viscosity except just below the SmA-Ne transition point T-AN, where an increase in the viscosity is observed. While in the Ne phase, an increase in the viscosity, which is strongly dependent on temperature, is observed upon application of the electric field; near T-AN a peak in the viscosity is observed below 1 KV mm(-1) followed by a gradual decrease to a constant value at higher electric field, whereas far from T-AN the viscosity increases monotonically with the electric field and saturates at high enough electric field. These results are discussed on the basis of the orientational change of the director and/or the smectic layer.