The ultra-fine magnesium oxide powders were obtained via direct precipitation process, during which the industrial pure MgSO4·7H2O( origined from szaibelyite) and (NH4)2C2O4 were emplyed as raw materials. The precursor and the final product Nano-meter MgO powders were characterized by IR, XRD and TEM. It belongs to cubic lattic, in sphere shape, white, stable in air. The diameter of powder is about 25-35 nm and homogenous. The mechanism and factors that influences the quality of final produce, such as reaction concentration, feeding fashion, calcining temperature were investigated.