Ba[(Zn1-xMgx)(1/3)Nb-2/3]O-3 ceramics were synthesized by the solid-state reaction process, and their microstructure and microwave dielectric properties were investigated. Surface secondary phase Ba5Nb4O15 due to the ZnO loss during sintering was detected for all the compositions except for x=1.0. Mg2+ substitution increased the order-disorder transition temperature and all the Mg2+-substituted ceramics were well ordered at a rather high sintering temperature. The dielectric constant and tau(f) decreased approximately in the same linear trend except for x=0.6 and 0.8, while the Q(f) values for the as-sintered and annealed ceramics indicated nonlinear variations with compositions inconsistent with the variation of cation-ordering degree. These results suggested that Q value in the ordered perovskite ceramics was determined not only by the cation-ordering degree but also by the domain size and its distribution, and the homogeneous domain structure with domain size around 40 nm should be critical for high Q. Good combination of microwave dielectric properties was obtained in the as-sintered ceramics with x=0.4: epsilon(r)=36, Q(f)=94 400 GHz, and tau(f)=28.6 ppm/degrees C.