In this paper a new laser system with an SBS phase conjugating mirror ig described. It consists of a Nd: YAG laser oscillator, a laser amplifier, and an SBS phaes conjugating mirror. A polarization discrimination technique is used to completely couple out the backward conjugating wave from the laser system. Effects of Brillouin cell geometry have been studied experimentally. By using acetone (300mm in length) as the scattering medium, Q-switched laser output of 100 mJ which compensated phase distortion has been obtained.