A simple method is presented for the determination of the rotational viscosity of nematic liquid crystals. Theoretical expressions for the rotational viscosity from transient displacement current in nematic liquid crystals are derived. The validity of the expressions is numerically examined. It is found that the rotational viscosity can be obtained with sufficient accuracy. The rotational viscosity of ZLI-2293 (Merck Ltd.) is measured at various temperatures by the present method.