The experimental and theoretical study of the electrorheological (ER) effect observed in the nematic phase of the 4-n-hepthyl-4'-cyanobiphenyl (K21) is the aim of this work. The K21 liquid crystal (LC) appears to be a model system where all the observed rheologial behaviours can be interpreted by the Leslie-Ericksen (L-E) continuum theory for low molecular weight liquid crystals. We present the flow curves of our sample for different temperatures and under the influence of an external electric field, ranging from 0 to 3kV/mm, applied perpendicular to the flow direction. We also present the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. A master flow curve was built, dividing the shear rate by the square of the electric field and multiplying by the square of a reference electric field value, for each temperature, where two Newtonian plateaus appear at low and high shear rate values, connected by a shear-thinning region at intermediate shear rate values. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory. In this description the director alignment angle is a function of the electric field and the flow field - boundary conditions are neglected. In this way it was possible to extract some viscoelastic parameters, as well as the dielectric anisotropy.