The effect of BaWO4 addition on the microstructures, the phase formation, 1:2 ordering and microwave dielectric properties of Ba(Mg1/3Nb2/3)O-3 was investigated. Ba5Nb4O15 and BaWO4 secondary phases were found in all specimens containing BaWO4. The B-site ordering parameter (S) and the lattice constant (c/a) values increased with BaWO4 increased. Ba(Mg1/3Nb2/3)O-3 ceramics with BaWO4 addition can be sintered to 95% theoretical density at 1500degreesC due to the liquid phase effect of BaWO4. At 0.03 mol BaWO4, the specimen shows the maximum Qf value of 82300GHz attributed to high density and ordering degree. The decrease of Qf value with further addition of BaWO4 is due to the increase of secondary phases BaWO4 and Ba5Nb4O15, which have lower Qf values than Ba(Mg1/3Nb2/3)O-3 phase. The variation of epsilon(tau) and tau(f) was also explained based on the logarithmic mixture rule and the volume mixture ruler, respectively. (C) 2004 Elsevier B.V. All rights reserved.