MgO-doped Ba0.55Sr0.45TiO3 ceramics were prepared by solid-state reaction technique and their phase compositions, microstructure and dielectric properties were systematically characterized. Their dielectric peaks were markedly suppressed, broadened, and shifted to low temperatures with increasing MgO content, indicating that a certain amount of Mg2+ incorporated into BST lattice. The reason why the loss tangent at microwave frequencies increased with the addition of MgO at low concentrations was identified. Raman spectra analysis clearly indicated that the incorporation of Mg2+ degraded quality factor (Q value) of BST, which was ascribed to the deterioration of B-site ordering of the ABO(3) perovskite structure. It was found that Q value was strongly correlated with the Raman peak width of A1 (TO) stretch modes in perovskite materials. Also oxygen octahedron played an important role in determining the material's microwave performance. The decrease in loss tangent at microwave frequencies of the samples with high contents of MgO was resulted from the "composite" effect of the dielectric component.