The instability and multi-mode oscillation of the output caused by depolarization of the linearly polarized light into the ellipti cally polarized light in transmission due to thermo-induced birefringence of th e YAG rode are studied. Electrooptical Q switching technology is applied to the Nd:YAG laser pumped by a CW Kr lamp. To generate stable and single -mode output, a pair of λ/4 plates are arranged in the oscillator. Electr ooptical Q-switching at a repetition of 1～5 kHz and a narrow-pulse-duration laser output are experimenttally obtained. A novel electrooptical Q-switching technique is provided for CW Nd:YAG lasers.