B2O3-doped NiTiNb2O8 microwave dielectric ceramics were prepared via the conventional solid-state reaction route. The effect of B2O3 additives on the sintering behavior, microstructure and microwave dielectric properties were investigated systematically. The addition of B2O3 as liquid phase sintering aid lowered the sintering temperature of NiTiNb2O8 ceramics successfully from 1250 to 1000 °C. With 1 wt% B2O3 addition, the system remained single-phase NiTiNb2O8, and the second phase NbBO4 was observed with 2–4 wt% B2O3 addition. From the study on the sintering behavior of the NiTiNb2O8 ceramics, 2 wt% B2O3 addition made the relative density reached the maximum value at 1000 °C. The 2 wt% doping of B2O3 had a positive effect on the properties of the NiTiNb2O8 ceramics: the $${\varepsilon _r}$$ εr value increased from 62.62 to 64.27, the Q × f value increased from 7294 to 12531 GHz while the τf values decreased from + 128 to + 111 ppm/°C.