<p>Transparent piezoelectric ceramic Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) can couple several physical effects such as acoustics, optics, mechanics and electricity, which can be used as mechanical-electro-optical multifunctional devices. The diffuse phase transition induced by composition has a significant impact on piezoelectric/ferroelectric properties. In this paper, we found a dramatic influence of Pr on the diffusion phase transition and electric properties of 28PIN-38PMN-34PT-<i>x</i>Pr. A phase coexistence of rhombohedral and tetragonal with trace of pyrochlore phase appears in the system, and the fraction of rhombohedral phase increases firstly and then decreases with the increase of Pr. Temperature dependence of dielectric permittivity exhibits a clear change from diffusion phase transition behavior (0%≦<i>x</i> ≦ 1.5%) to frequency dispersion behavior (2%≦<i>x</i> ≦ 3.5%). The size of polar nanoregions (PNRs) remains unchanged for <i>x</i> ≦ 1.5% characterized by the phenomenological statistical model, and it decreases first and then increases for <i>x</i> ≧ 1.5%, which is consistent with the trend of spontaneous polarization and piezoelectric coefficient having the same change with PNRs. This work gives an insight on the relationship between electrical properties and phase structure modified by composition in perovskite oxides.</p>