Abstract An optical resonance method for measuring ion energy has been suggested. The experimental details are described. The method relates the resonant oprical transition of ions and the ion energy through the Doppler effect. The method can be applied in the energy range of leV-100 MeV, and its accuracy is expreted to reach 5×10-5 near energy of 30 keV. We have measured the 142Nd+-ion energy by this method and the result is fairly good.