Dielectric constants, DSC(differential scanning calorimeter), X-ray diffraction studies of pseudobinary $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3-xPb(Yb_{1/2}Ta_{1/2})O_3$ ceramic system were carried. X-ray structure studies at room temperature show that pure $Pb(Yb_{1/2}Ta_{1/2})O_3$ is not a single perovskite phase but contains small amount of pyrochlore phase. The lattice parameters and Curie temperature show no change in the whole composition range. This is due to the similarity in ionic size of $Ta^{+5}$ and $Nb^{+5}$. Though long range order parameter(S) of B site ordering decrease from 0.9 to 0.82 as x increases up to 0.35, high degree of long range order is well observed. The dielectric constants as a function of temperature and measuring frequency confirm Debye type relaxation. First order phase transition is revealed since no change of Tc according to frequency and strong thermal hysteresis of dielectric constant and large latent heat at phase transition temperature.
