Nano-powder of magnesium oxide was prepared by direct precipitation method using MgCl2?6H2 O as raw material, Na2 CO3 aqueous solution as precipitant and ethylene glycol as medium. The calcining temperature range of precursor was deter-mined by TG analysis. The activity of calcined products was measured by the citric acid method and batch defluoridation then com-pared their activity with the analytically pure magnesium oxide. The average crystallite size of calcined products was calculated by X-ray diffraction method. Effect of calcining temperature and holding time on the crystallite size and activity of Nano-powder of mag-nesium oxide were investigated. The results indicated that Nano-powder of magnesium oxide showed the highest activity(CAA value was 14. 35s)and smaller crystallite size(15. 77nm)when the calcining temperature was about 430℃ and the holding time was 1. 5 hour while it possessed higher activity(CAA value was 23. 44s)and the smallest crystallite size(11. 47nm)at about 450℃ for 1. 5 hour. It can clearly be seen that calcining processes have great effect on the crystallite size and activity of Nano-powder of magnesi-um oxide. Superfine magnesium oxide is not necessarily extra light calcined magnesia. In addition,the paper also discussed the mechanism of basic magnesium carbonate for low temperature decomposition.%以六水合氯化镁和碳酸钠为原料，乙二醇为介质，采用直接沉淀法制备了纳米 MgO粉体。通过热重分析确定了前驱物的煅烧温度范围，采用柠檬酸值法、静态除氟测定了煅烧产物的活性，并与普通MgO的活性进行了比较，X射线衍射法计算了煅烧产物的平均粒径，探讨了煅烧温度和保温时间对纳米MgO粒径及活性的影响。结果表明：当煅烧温度为430℃和保温时间为1.5h时，得到的纳米 MgO活性最高（CAA值为14.35s），粒径较小15.77nm；而当煅烧温度为450℃和保温时间为1.5h时得到的纳米MgO活性较高（CAA值为23.44 s），粒径最小11.47nm。可见，煅烧工艺对纳米MgO粒径及活性影响很大，超细MgO并不一定是高活性MgO。另外，探讨了碱式碳酸镁低温分解机理。
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