Flaky and porous rod-like MgO powders were prepared by molten salt synthesis with MgCl2, CaCO3, NaNO3 and KNO3 as raw materials. TG-DSC, XRD and SEM were used to investigate the reacion process and characterize the resulting products. The results indicated that Ca2+ was displaced by Mg2+ and dolomites were formed and finally converted into magnesium carbonates during the molten salt synthesis. Fibrous Mg(OH)2 formed in PEG solution and flaky Mg(OH)2 formed in EDTA-PEG solution illustrated that EDTA and PEG molecules could define the growth direction of Mg(OH)2. The porous rod-like MgO powders with 20-80 nm in diameter and 40-100 in length-to-width ratio were obtained by the decomposition of fibrous Mg(OH)2 at 600 °C, while the flaky MgO powders were prepared by the decomposition of flaky Mg(OH)2 at 600 °C. Flaky MgO powders were platelike, with 30-50 nm in thickness, 200-500 nm in diameter, and up to 1.0 µm2 in a single slice surface area.