The Microstructure and microwave dielectric properties of Bi2O3-deficient Bi12SiO20 ceramics were investigated. A small amount of unreacted Bi2O3 phase melted during sintering at 825°C and assisted with densification and grain growth in all samples. The melted Bi2O3 reacted with remnant SiO2 during cooling to form a Bi4Si3O12 secondary phase. The nominal composition of Bi11.8SiO19.7 ceramics sintered at 825°C for 4 h had a high relative density of 97% of the theoretical density, and good microwave dielectric properties: εr = 39, Q × f = 74 000 GHz, and τf = −14.1 ppm/°C. Moreover, this ceramic did not react with Ag at 825°C.