Ba(Ti1-xMnx)O-3 ceramics were synthesized by a traditional solid-state reaction. The Ba(Ti1-xMnx)O-3 samples with x=0.15, 0.3, 0.4, and 0.5 of MnO, Mn2O3, and Mn3O4 were sintered at 1450 degrees C for 2 h, then cooled to room temperature to obtain hexagonal barium titanate. The microstructures and dielectric properties of the Ba(Ti1-xMnx)O-3 samples were analyzed by dielectric property measurement, X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy (TEM). The results showed that the lattice parameters decrease as the x value increases from 0.15 to 0.5, and a secondary phase, which is similar to BaFeO3 structure (JCPDS 74-0646), was detected at x=0.5. With an increasing x, the grains developed a plate-like morphology. For samples with MnO additive, the secondary phase, which is similar to Ba2TiO4 structure (JCPDS 38-1481), was detected on the grain boundaries due to the excess of divalent Ba ions. For samples with Mn2O3 and Mn3O4 additives, the liquid phase and nanocrystalline Mn2O3 grains were detected due to the excess of Ti and Mn atoms. For dielectric properties, the quality factor (Q x f ) reaches the maximum value (18 344 GHz) at x=0.3 for the addition of Mn3O4. The TEM analysis was focused on the Ba(Ti0.5Mn0.5)O-3 specimens in order to identify the relationship between the 6H- and 12R-structures, and their formation mechanism.