Microwave dielectric ceramics based on monoclinic-structure ZnZrNb2O8 were prepared by an aqueous sol–gel process. Highly reactive nanosized ZnZrNb2O8 powders with particle sizes of 10–30 nm were successfully obtained at 600 °C as precursors. And the evolution of phase formation was investigated by means of DTA–TG and XRD analysis. Both selected area electronic diffraction patterns and high-resolution crystal lattice image were characterized for the evaluation of crystallinity by mean of transmission electron microscopy. Sintering characteristics and microwave dielectric properties of ZnZrNb2O8 ceramics were studied depending on sintering temperatures ranging from 1100 to 1250 °C. The dielectric properties were strongly dependent on the compositions, the densifications and the microstructures of the specimens. With the increase of sintering temperature, density, εr and Q·f values increased and saturated at 1200 °C with excellent microwave properties: ɛ r of 27.38, Q·f value of 66,700 GHz, and τ f value of −38.4 ppm/°C. The correlations of microstructure and dielectric properties for ZnZrNb2O8 ceramics were also investigated.