Phase relations of the Nb2O5–CaO–MgO ternary system in equilibriumEquilibrium at 1523 K in air have been investigated by the high-temperatureTemperature equilibration/quenching method. The compositions of equilibriumEquilibrium phase were determined by Scanning Electron MicroscopyScanning Electron Microscopy (SEM) with Energy Dispersive SpectroscopyEnergy dispersive spectroscopy (SEMScanning Electron Microscopy (SEM)-EDS) and X-ray diffraction (XRDX-Ray Diffraction (XRD)). Seven sub-equilibriumEquilibrium regions in the Nb2O5–CaO–MgO system were measured at 1523 K in air, including Ca3MgNb2O9–MgO–CaO region, Ca3MgNb2O9–Ca2Nb2O7–Mg4Nb2O9 region, CaNb2O6–Mg5Nb4O15–MgNb2O6 region, CaNb2O6–Nb2O5–MgNb2O6 region, Ca2Nb2O7–CaNb2O6–Mg4Nb2O9 region, CaNb2O6–Mg5Nb4O15–Mg4Nb2O9 region, and Ca3MgNb2O9–Mg4Nb2O9–MgO region. Within the experimentally investigated composition range, a ternary compound, Ca3MgNb2O9, is present in the system. The solid solubilitySolubility of MgO is 6.36 mol%, 6.12 mol%, and 4.21 mol%. The MgO phase, the MgNb2O6 phase, the Mg4Nb2O9, and the Mg5Nb4O15 phases can dissolve CaO. The solid solubilitySolubility of CaO is 2.72 mol%, 3.70 mol%, 1.28 mol%, and 0.60 mol%. The determination of the Nb2O5–CaO–MgO ternary system further enriches the thermodynamicThermodynamic databases of silicates and niobates.