Group VIII metal oxides, that is, Fe2O3, Co2O3, and NiO have been introduced to 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr0.50Ti0.50)O3 (PZN–PZT) to deterministically identify the substitution mechanism and meantime to tailor mechanical and piezoelectric properties in obtaining energy harvesting materials. On the basis of the X-ray diffraction and Raman analysis, it is clear that the group VIII metal oxides induce a phase transformation from the morphotropic phase boundary to the tetragonal phase side, and the corresponding grain size increases accordingly. It is reasonable to deduce that two types of substitution behaviors coexist in the group VIII metal oxides added PZN–PZT system. Due to the mixed valence of +2 and +3, the foreign doping ions prefer to enter the B site in the perovskite structure, not only substituting for Ti4+, Zr4+, and Nb5+ ions in the inequivalence replacement but also substituting for Zn2+ ions in the equivalence replacement. The proposed complex substitution mechanisms can give the full explanation about the grain growth phenomena and the variation in mechanical and electric properties in the modified PZN–PZT system. At the same doping level of 0.3 mol%, the maximum transduction coefficient (d33·g33 = 13120 × 10−15 m2/N) and good fracture toughness (KIC = 1.32 MPa m1/2) are obtained in Co2O3 added 0.2PZN–0.8PZT ceramics, which shows great promise as practical materials for energy harvesting device applications.