The sintering temperature of BaSm 2Ti 4O 12 (BST) and BaNd 2Ti 5O 14 (BNT) ceramics was approximately 1350 °C and decreased to 875 °C with the addition of BaCu(B 2O 5) (BCB) ceramic powder. The presence of the liquid phase was responsible for the decrease of the sintering temperature. The liquid phase is considered to have a composition similar to the BaO-deficient BCB. The bulk density and dielectric constant ( ɛ r) of the specimens increased and reached saturated value with increasing BCB content. The Q-value initially increased with the addition of BCB, but decreased considerably when a large amount of BCB was added, because of the presence of the liquid phase. Good microwave dielectric properties of Q × f = 4500 GHz, ɛ r = 60 and τ f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h. Moreover, the BST and BNT ceramics containing BCB show good compatibility with silver metal.