B2O3 added Ba(Mg1/3Nb2/3)O-3 (BBMN) ceramics cannot be sintered below 930 degrees C. However, when CuO was added to them, they were sintered even at 850 T. The amount of the Ba2B2O5 second phase, which was formed in the BBMN ceramics decreased with the addition of CuO. Therefore, the CuO additive is considered to react with the B2O3 inhibiting the reaction between B2O3 and BaO. A dense microstructure without pores developed with the addition of a small amount of CuO. The bulk density, dielectric constant (epsilon(r)) and Q-value increased with the addition of CuO, but decreased when a large amount of CuO was added. Excellent microwave dielectric properties were obtained for the Ba(Mg1/3Nb2/3)O-3 + 2.0 mol% B2O3 + 10.0 mol% CuO ceramic sintered at 875 T for 2 h, with values Q(xf) = 21500 GHz, epsilon(r) = 31 and temperature coefficient of resonance frequency (tau(f)) = 21.3 ppm/degrees C. (c) 2005 Elsevier Ltd. All rights reserved.