Effects of Mg-substitution on the structure, microstructure and microwave dielectric properties of Ba [(Ni0.6Zn0.4)(1/3)Nb-2/3]O-3 ceramics have been systematically investigated. Mg-substitution enhances the densification and promotes the homogenization of grain distribution. Single phase of 1:2 ordered complex perovskite structure forms in the entire composition range of Ba[(Ni0.6-x/2Zn0.4-x/2Mgx)(1/3)Nb-2/3] O-3 ceramics. Mg-substitution increases the 1:2 cations ordering degree and the ordering domain size. With increasing x, the Q x f value increases significantly, a slight decrease occurs in epsilon(r), and, tau(f) undergoes a small positive shift. Raman spectra reveal that Mg-substitution enhances the coherency of the A(1g) (O) vibration mode and thus improves the Q x f value. Post-densification annealing has been performed to further increase the cations ordering degree and improve the microwave dielectric properties for x = 0.3. With increasing the annealing temperature, the ordering degree increases monotonically, while the Q x f value increases first and then decreases, meanwhile epsilon(r) decreases slightly and tau(f) exhibits a negative shift. A good combination of microwave dielectric properties has been achieved for x = 0.3: epsilon(r) = 32.0, Q x f= 56,500 GHz, tau(f) = 8.4 ppm/degrees C. (C) 2015 Elsevier B.V. All rights reserved.