Abstract: A pot experiment was carried out to study the malic acid(0,1,2 and 3g/L) on nitrogen metabolism of flue?cured tobacco seedling after 20 d and 40 d transplantation.Three plants were collected from each treatment to test physiological characters and chemical components after 50 d transplantation.The result showed the appropriate amount of malic acid promoted the release of nitrogen in soil and accelerated the accumulation of nitrogen in plant.The root vitality was increased and nitrogen metabolism was strengthened with the increasing of malic acid concentration in terms of the promoted nitrate reductase activity,reduced content of nitrate nitrogen and the increased of content of ammonium nitrogen in root,while the quantity of nitrogen was reduced in terms of reduction of indicates that nitrate nitrogen and nitrate reductase.We can conclude that applying malic acid altered the status of the nitrogen metabolism in the leaf and root. It could increase the proportion of reduction and assimilation in the root and decrease the proportion of reduction in the leaf.In general,malic acid application promoted the nitrogen metabolism of flue-cured tobacco.