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 Pmc2(1) and cell parameters a = 15.6999(9), b = 5.5521(1), and c = 5.5830(1) angstrom. 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.