Abstract: The effect of magnesium deficiency on nitrogen metabolism of longan (Dimocarpus longanaLour) seedlings was studied using solution culture In leaves with Mg deficient , the activity of nitrate reductase,the total amount of hydrolytic amino acids decreased,whereas the total amount of free amino acids increased.With the increase in severity of Mg deficiency,the activities of RNase and protease in leaves increased markedly.As a result, the degradation of RNA and protein were promoted and their contents reduced; the content of DNA,to a less extent than RNA, also declined.Furthermore, the net rate of O2- generation increased in Mg deficiency leaves Statistics analysis showed that the decrease of nucleic acid and protein in Mg deficient longan leaves were the comprehensive results of synthesis retardation, degradation aggravation and active oxygen stress.