Abstract: The precursor of MgO was prepared by direct precipitation method with different conditions，such as changing temperatures，concentrations，calcination times，and surfactants，with magnesium chloride hexahydrate and ammonium carbonate as raw materials.MgO particles were prepared by calcining the precursor in the absence of air.The particle size distributions of the products prepared under different conditions were measured by laser particle analyzer.The different conditions and the changing rule of the particle size of the product were summarized.The decomposition temperature of precursor was confirmed by TG-DSC.The products were characterized by XRD and SEM.Results showed under the optimum conditions，the nano-sized MgO with average particle size of about 50 nm could be prepared and the morphology was dumbbell-shaped.

Key words: magnesium oxide, particle size, direct precipitation method