A melilite Ba2CuGe2O7 ceramic was characterized with low sintering temperature and moderate microwave dielectric properties. Sintered at 960 oC, the Ba2CuGe2O7 ceramic had a high relative density 97%, a low relative permittivity 9.43, and a quality factor of 20,000 GHz. To get a deep understanding of the relationship between composition, structure, and dielectric performances, magnesium substitution for copper in Ba2CuGe2O7 was conducted. Influences of magnesium doping on the sintering behavior, crystal structure, and microwave dielectric properties were studied. Mg doping in Ba2CuGe2O7 caused negligible changes in the macroscopic crystal structure, grain morphology, and size distribution, while induced visible variation in the local structure as revealed by Raman analysis. Microwave dielectric properties exhibit a remarkable dependence on composition. On increasing the magnesium content, the dielectric constant (εr) featured a continuous decrease, while both the quality factor (Q × f) and the temperature coefficient of resonance frequency (τf) increased monotonously. Such variations in dielectric performances were clarified in terms of the polarizability, packing fraction, and band valence theory.