In this work, the piezoelectric ceramic system of Pb[(Zr1-xTix)(0.74)(Mg1/3Nb2/3)(0.20)(Zn1/3Nb2/3)(0.06)]O-3, 0.47 less than or equal to x less than or equal to 0.57, with composition close to the morphotropic phase boundary, was studied. From the results of X-ray diffraction and piezoelectric measurement, ceramics near x = 0.51 were found at the morphotropic phase boundary (MPB) between the tetragonal and pseudocubic perovskite. The planar coupling factor (k(p) = 0.72) is high at compositions near the MPB, but the mechanical quality factor (Q(m) = 75) is low. The calculation of the diffuseness of phase transition shows that the region of phase coexistence of this system is broader than that of the ternary system. (C) 2004 Elsevier Ltd. All rights reserved.