$0.5Pb(Yb_{0.5}Nb_{0.5})O_3-0.5PbTiO_3[PYN_{0.5}PT_{0.5}]$ is a morphotropic mixture of pseudocubic and tetragonal phases. In this work, the dielectric, piezoelectric and phase transition behavior have been investigated when $PbZrO_3[PZ]$ is substituted near morphotropic $PYN_{0.5}PT_{0.5} [(PYN_{0.5}PT_{0.5})_{1-x}-PZ_x]$ for 0.00 ≤x ≤0.40. In the XRD measurement, the split ferroelectric (400) lines, which is indicative of the structural change, is found to merge into a single cubic line on PZ substitution. The crystal structure of this composition changes from the MPB(pseudocubic and tetragonal phases) to a pseudocubic phase with further $Zr^{4+}$ substitution for x>0.10. The dielectric constant has a maximum value at x=0.05 and its maximum value of the dielectric constant maximum decreases gradually with PZ substitution higher than x>0.05 while the apparent phase transition temperature decreases gradually for the whole composition range 0.00 ≤x ≤0.40. $PYN_{0.5}PT_{0.5}$ undergoes a phase transition well defined by the distinct transition temperature. For the ternary solid solutions of x=0.05 and 0.10, a broad secondary shoulder shows up below the transition temperature in the dielectric constant vs. temperature curve after poling under 30kV/cm at 100℃. The remanent polarization reaches the maximum value $(Pr = 39mC/ ㎠)$ at x=0.05. It decreases, however, with higher PZ substitution for x>0.05. The piezoelectric constant ($d_{33}$) also has the maximum value(~215pC/N) at x=0.05. In addition, morphotropic phase boundary of Zr-substituted $PYN_{0.5}PT_{0.5}$ has been calculated using Landau-Devonshire free energy represented by polynomials of the order parameter(spontaneous polarization).
