Rheological properties in the nematic phase of 5CB (4-n-pentyl-4’-cyanobiphenyl) an investigated under applications of electric fields. When an electric field of a few kV mm(-1) is applied, an increase in the apparent viscosity is observed with its amount being saturated at higher electric fields, which is accompanied by a change of the how from a Newtonian to a non-Newtonian and again to a Newtonian fluid. Such an electrorheological effect is observed only in the nematic phase with the effect being dependent on shear rate and temperature. On the basis of the theory of Leslie and Ericksen, the electric field dependence of the viscosity is calculated as a function of shear rate in good quantitative agreement with the experimental one. These results show that the observed viscosity change is due to a change of the director orientation, determined by the balance between the electric and the viscous torques exerted on the director.