Magnesium ferrite MgFe2O4 nanopowders were synthesized using chemical co-precipitation assisted with ultrasound irradiation technique. The ferrite precursors were precipitated from the corresponding sulfates by 5 M sodium hydroxide at pH 1.1 without and with ultrasound irradiation for 2-6 h. Thermal treatment of the produced precursors with and without ultrasound irradiation for 2 h at temperature ranged from 600 to 1,000 degrees C was also studied. The powders formed were characterized by X-ray diffraction (XRD), Fourier transformer infrared (FT-IR), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). Effects of ultrasound irradiation time, calcination temperature and Fe/Mg mole ratio on the phase formation and magnetic properties were reported. Pure crystalline MgFe2O4 phase can be obtained by thermal treatment of the precursor at temperature 1,000 degrees C with ultrasound irradiation for 2 h with high magnetization (18.39 emu/g).