Abstract: At varied nitrogen levels, apot experiment was conducted to study the effect of nitrogen on the contents of chlorophyll, invertase, soluble sugar, reducing sugar and starch, as well as the quality of the tobacco leaves for flue-curing.The results showed that the invertase and amylase activities at the mid-and late-tobacco growth stages increased with appropriate application of potassium, which also benefited the transition of nitrogen to carbon metabolism at the right time period.After 30and 55ds of transplantation, the contents of total soluble sugar, reducing sugar and starch in the tobacco leaves decreased with increasing potassium fertilization at the early stage of tobacco.They increased with the increased application of potassium at the mid-and late-stages after 75ds of the transplanting, suggesting the application of potassium would be better for carbon accumulation at the late, not the early stage of tobacco growth.When nitrogen supply was insufficient, the growth of tobacco was slow, the photosynthetic pigments low, and the leaf color yellowish.When excess nitrogen was present, the tobacco grew vigorously and delayed the normal leaf-falling.Under the experimental conditions, the optimal levels of nitrogen for the paddy field soil (available nitrogen 184.31mg·kg-1) , the yellow mud field soil (available nitrogen 127.27mg·kg-1) , and the tidal field soil (available nitrogen 105.45mg·kg-1) were 0.90g·pot-1, 1.35-1.90g·pot-1, and 1.90g·pot-1, respectively, which resulted in the highest yield and the most balanced and desirable quality for the tobacco.