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Barium titanate-based microwave dielectrics usually require relatively high temperatures to sinter, which prevents the use of base metals such as copper for electrodes. In this work, BaTi4O9 microwave dielectric ceramics co-fired with copper electrodes are made possible by adding B-Si-Ba- Zn-O glass to induce liquid-phase sintering at sufficiently low temperature and in reduced atmosphere. The microstructures and electric properties of the BaTi4O9 ceramics thus obtained are carefully examined and studied. Proper glass composition may significantly facilitate mass transportation in the low-temperature co-fired ceramic (LTCC) material, resulting in better densification without serious degradation of electric properties. Although the B2O3/SiO2 ratio enhances the glass mobility during sintering, the BaO/ZnO ratio contributes to the chemical affinity of glass to BaTi4O9 ceramics. In addition, various Ba-Ti-O phases with different Ba/Ti ratios may be found in the specimen through the X-ray diffraction patterns when the BaO/ZnO ratio is varied. If the BaO/ZnO ratio is high and the glass flows easily in the material, the Ba4Ti13O30 phase is formed. If the BaO/ZnO ratio is low and the glass flows easily in the material, the BaTi6O13 phase appears. We find that glass-induced Ba4Ti13O30 transformation may significantly decrease Qxf values in the BT4-BSBZ materials. Therefore, the appropriate glass composition must be selected to ensure the phase stability of dielectrics to achieve the best performance possible. [ABSTRACT FROM AUTHOR]