One of the best known solid solution of perovskites is the PbTiO(3)-PbZrO(3) system which is usually abbreviated as PZT. In the phase diagram of this system, there is a drastic phase change from tetragonal to rhombohedral at the molar ratio around 50:50. The PZT crystals are widely used as a practical piezoelectric material because of its very strong piezoelectric effect near this morphotropic phase boundary. We try to explain this anomaly in piezoelectric constant by a phenomenological theory.