The MgO ceramic powders have been prepared from decomposition of magnesium carbonate Mg5(CO3)4(OH2-4H2O (hydromagnesite ) and magnesium hydroxide Mg (OH)2 ( brucite ) at different decomposition temperatures and limes which have been characterized by XRD and DTA methods. The results of x-ray diffraction patterns for MgO confirm the existence of short-range order phase, in addition to the long-range order lines (111). (200), (220) and 420). The intensities of the shortrange order phase have been shown to fall with increasing temperature on favour of long-range order lines. Preliminary treatment of the broaded x-ray diffraction lines by Warren corrected method has shown that less accurate crystallite size is achieved. However, detailed treatment using deconvolution of ihe experimental profiles by Fourier cosine coefficients has resulted in corrected values of crystallite size, which have been found to increase linearly from 40-70 °A at 400 °C to 115-133 °A at 1500 °C. Strain on the other hand decreases linearly from 8.0 x 10-3 at 400 C to 1.6 x10'3 at I500 C Morphological details observed by SEM on the grains of magnesia shows the circular shape of small crystallite aggregation of MgO prepared from hydromagnesite and ellipsoid for MgO from brucite. Low temperature decomposition ( 400 °C ) of hydromagnesite yield active MgO powder of high surface area ( 411 m2/gm ). And this value drop drastically to 124 m2/gm at 1500 °C. Brucite on the other hand yield MgO powder of small variation in surface area ( 220-155 m2/gm )in this temperature range. Estimation of microhardness on MgO compacts of different decomposition temperature and crystallite size have sit shown that the crystallite size and the origin of reactant may influence the bearing load that compacts can endure; the hardness of compacts from hydromaguesite being twice that from brucite.