Microwave dielectric ceramics of Mg 4 Nb 2 O 9 were prepared by the sol–gel method and their microwave dielectric properties and microstructure were investigated in this study. Firstly, highly reactive nanosized magnesium niobate powders were successfully synthesized at 550°C in oxygen atmosphere with particle size of 30–40nm. And the phase formation was investigated by means of DTA–TG and XRD analyses. Subsequently, the sintering ability and microwave dielectric properties of Mg 4 Nb 2 O 9 ceramics were studied under various sintering temperatures ranging from 1150°C to 1325°C. As the sintering temperature increased from 1150°C to 1325°C, the density values, the ɛ r values and the Qf values increased and saturated at 1250°C, whereas the Qf values decreased at the sintering temperature above 1300°C .The τ f values were ranged from −48.7 to −37.8ppm/°C. Due to the increased density and appropriate grain growth the Mg 4 Nb 2 O 9 ceramics sintered at 1250°C had the excellent microwave dielectric properties of ɛ r =12.3, Qf=165,000GHz and τ f =−47.5ppm/°C. The results showed that the sintering temperature was significantly reduced by the sol–gel process.