Axis-cut LiNbO3 crystal piezoelectric strain optical modulators (PESOM)and its properties when they used as laser mode lockers are analysed in detail. We have proved that x-cut plate is the best form used for laser mode locker. And y-cut or z-cut plate will result in loss modulation and frequency modulation at the same time, so in principle they are not suitable for using as laser mode locker operating at steady state.