Niobate perovskite oxides of KNb1-xMgxO 3-δ were synthesized directly at high temperature and pressure. X-ray diffraction confirmed the formation of single-phase orthorhombic structures. High-temperature Raman spectroscopy in combination with thermal analysis showed that phase transitions occurred from orthorhombic to tetragonal, to pseudo cubic, and finally to cubic, in sequence. Addition of Mg2+ at the Nb5+ sites of perovskite KNbO3 lattice led to a dramatic enhancement of oxide-ion conduction. This enhancement in oxide-ion conduction was through the suppression of the contribution from p-type holes, which shows that these high-temperature phases can be promising candidates as conductive materials.