Cubic shaped Magnesium oxide nanoparticles were successfully synthesized by sol-gel method using magnesium nitrate and sodium hydroxide at room temperature. Hydrated Magnesium oxide nanoparticles were annealed in air at 300 and 500°C. X-ray diffraction patterns indicate that the obtain nanoparticles are in good crystallinity, pure magnesium oxide periclase phase with (200) orientation. Morphological investigation by FESEM reveals that the typical sizes of the grown nanoparticles are in the range of 50-70nm. Powder composition was analyzed by the FTIR spectroscopy and the results confirms that the conversion of brucite phase magnesium hydroxide in to magnesium oxide periclase phase was achieved at 300°C.The Thermo-gravimetric analysis showed the phase transition of the synthesized magnesium oxide nanoparticles occurs at 280-300°C.