The MgO powder is prepared by thermal decomposition of hydromagnesite of Mg5(CO3)4.(OH)2.4H2O and magnesium hydroxide Mg(OH)2 powders in the temperature range 400-1000°C. The peak full width at the ha!f maximum measured and corrected according to three different equations Deals with the shape of the diffraction peak. The crystallite size is then determined by using Scherror's equation. The crystallite size is found to increase linearly with temperature. Adding a binder (P.V.A) to the initial reactants assist the hydromagnesite to decomposed at lower temperature . The effect of adding Calcium carbonate to the hydromagnesite in air and in vacuum on the crystallite size of MgO is studied and it is found that the crystalline size of MgO obtained is smaller than crystallite size of MgO which is obtained without adding Calcium Carbonate which means that the CaCO3 act to inhibit the growth of MgO. Yet, Smaller crystallite size of MgO are observed when the decomposition of the mixtures of hydromagnesite and Calcium carbonate occurred in vacuum. Minimum crystallite size for Magnesium Oxide obtained after total converion of the initial reactants was 112.5 A° when CaCO3 is added and the decomposition take place in vacuum.