Thin films of MgO have been deposited on Si(100) and c-plane sapphire substrates by the metal-organic chemical vapor deposition (MOCVD) method using Mg(tmhd)(2) and Mg(acac)(2) as single molecular precursors and oxygen as carrier gas. We have synthesized the metal-organic precursors used in this study. Strongly [111] oriented polycrystalline MgO films were obtained on both Si(100) and c-plane sapphire substrates using these precursors in the temperature range 500-600 degrees C. The MgO thin films grown on Si(100) at 600 degrees C and on c-plane sapphire at 500 degrees C with Mg(tmhd)(2) are highly oriented in the [111] direction. whereas the MgO film grown on Si(100) at 350 degrees C and then annealed at 520 degrees C has no preferred orientation. In the case of using Mg(acac)(2) as precursor, however, the MgO film deposited on c-plane sapphire surface at a deposition temperature above 500 degrees C was grown with more [110] and [100] dominant orientation relatively rather than [111]. Furthermore, we have also shown that the synthesized precursors Mg(tmhd)(2) and Mg(acac)(2) are suitable precursors for obtaining MgO thin films by MOCVD and the substrate or precursor type and the growth temperature will be important factors influencing either the crystal growth direction or the crystallinity of the films.