The lead zirconate titanate (PZT) ceramics were prepared for the study of the relation between breakdown and partial discharge. $Pb(Zr_xTi_{1-x})O_3$(x=0.3, 0.4, 0.5), which have tetragonal phase were sintered at 1270-1280℃ for 1 hour. The breakdown strength and the partial discharge characteristics in the tetragonal phase PZT were investigated for various compositions. The breakdown strength decreases with increasing the amount of Ti. The Partial discharge in pores was observed under high ac voltage. The breakdown strength and the partial discharge characteristics were investigated after poling. In this case the breakdown strength and the partial discharge inception voltage were reduced than before poling. As a result, the mechanism of PZT breakdown seemed to be the accelerating increase of current due to the partial discharge at pores.