The phase-pure AgNbO3 has been prepared through the hydrothermal reactions of NH4HF2, Ag2O, and Nb2O5. The room-temperature powder X-ray diffraction analysis reveals that it crystallizes in orthorhombic perovskite lattice with a space group of Pmc21 and cell parameters = 15.6999(9), = 5.5521(1), and = 5.5830(1) Å. The temperatures, pH values, and times play crucial roles in obtaining phase-pure silver niobate. The structural phase transitions were observed at 813 K from cubic to tetrahedral phase and to final orthorhombic phase at 718 K, confirmed by TGA-DTA.