This study investigates the effect of CuO on the sintering behavior, dielectric properties, and microstructures of Ba0.6Sr0.4TiO3 (BST) ceramics. The ceramics were sintered in air at temperatures ranging from 1000 degrees to 1230 degrees C. It is found that a small amount of added CuO (0.6 mol%) can significantly increase the density and improve the dielectric properties of BST ceramics. Doped BST ceramics can be sintered to a density > 95% of the theoretical density at 1150 degrees C. scanning electron microscopic observations show that the BST grain sizes increase with increasing amounts of CuO. No secondary phases in the BST ceramics are observed using X-ray diffraction pattern for CuO additions up to 0.9 mol%. However, compositional analysis using transmission electron microscopy-EDX for the BST ceramics with 0.9 mol% CuO sintered at 1150 degrees C showed that a small level of secondary phase formation is present. On the other hand, large dislocations are observed for BST with 0.6 mol% CuO addition as a result of lattice distortion, which creates the vacancy condensation because of the atomic mismatch in the solid solutions. Optimal CuO doping concentrations can reduce the loss tangents of BST that can also ensure a high dielectric constant. When the doping concentration of CuO is 0.6 mol% and the ceramic is sintered at 1150 degrees C, the BST ceramic has the following properties at 1 MHz: dielectric constant=4094, tan delta=0.55%.