We present a simulation study concerning an optical modulator using the electrooptic (EO) Pockels effect and the prism excitation of a long-range surface plasmon (LSP) at a metal (Ag) - EO medium (CuCl) interface. It is demonstrated that by a proper optimization of the structure paramaters, LSP presents a very narrow ATR resonance. It is then possible to predict a voltage command less than 5 V for a 80 % modulation.