A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn3Nb2O8 powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn3Nb2O8 from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb2O6. Maximum sintering activity was observed for single-phase crystalline Zn3Nb2O8 powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V2O5 sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q×f product up to 36 000 GHz at sintering temperature (Ts)≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics.