The roles of additives (MnO2, SiO2, and CdO) in controlling microstructural development and piezoelectric properties of Pb(Ni1/3Nb2/3)O3– PbTiO3–PbZrO3(PNN–PT–PZ) ceramics were systematically examined. The addition of SiO2 (<1 wt. %) to the pseudoternary PNN–PT-PZ system enhances densification, but suppresses grain growth significantly. On the other hand, the presence of MnO2 expedites the rate of grain growth without increasing the rate of densification appreciably. The observed difference in the grain-growth behavior was discussed in terms of the viscosity of liquid phase formed during sintering and the diffusion-controlled process for the solute-atoms transport. The rapid increase in the mechanical quality factor (Qm) and the decrease in the relative dielectric permittivity (∊r) and d33 for the MnO2-doped specimens indicate the formation of oxygen vacancies by the dissolution of Mn-atoms into the B-sites of perovskite structure. On the contrary, the presence of CdO (2 mol %) remarkably increases ∊r and d33 of PNN–PT–PZ ceramics.