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Effect of SnO 2 addition on the crystal structure/ microstructure and the related microwave dielectric properties of the Ba 2 Ti 9 O 20 were systematically investigated. Incorporation of SnO 2 markedly stabilized the phase constituent and microstructure for the Ba 2 Ti 9 O 20 such that high quality materials can be obtained in a much wider processing window. The sintered density of the Ba 2 Ti 9 O 20 increased linearly, but the microwave dielectric constant (K) decreased monotonically, with the SnO 2 doping concentration. The quality factor (Qxf) of the materials increased firstly due to the addition of SnO 2 , but decreased slightly with further increase in SnO 2 content. The best microwave dielectric properties obtained are K =38.5 and Qxf= 31,500 GHz, which occurs for the 0.055 mol SnO 2 -doped and 1350°C/4 h sintered samples. These properties are markedly better than those for undoped materials (K=38.8 and Qxf=26,500 GHz).