With the buried glass filter paper bag method a field experiment was carried out in a tobacco field to study mineralization of green manure and its effect on tobacco leaf quality. Results show that about 81% of the organic C and 54% of the organic N in alfalfa, 63% of the organic C and 22% of the organic N in rye grass were mineralized throughout the growth period of tobacco. However, more than 92% of the mineralized N in alfalfa and 74% in rye grass were released in the first 6 weeks after they were incorporated into the soil. In the whole growth period of the plant, about N 43 kg hm^-2 was released from alfalfa. In order to control of over-supply of N nutrient in the late growth period and resultant higher nicotine content in tobacco leaves at upper stalk position, it is recommended that alfalfa should be ploughed into the field earlier and the total N fertilizer application rate should be reduced by deducting the amount of mineralized N released from the alfalfa. Rye grass released only N 10.5 kg hm^-2 in the same period of time because of its low N content. So rye grass could totally be ploughed and incorporated into the soil unless its biomass production is getting too high. When the application rate of chemical N fertilizer remained unchanged, the alfalfa treatment resulted in slightly higher nicotine content, lower total sugar and lower reducing sugar content in tobacco leaves at the upper stalk position in this experiment, while the rye grass treatment reduced the nicotine content and kept sugar contents in a proper range, thus improving quality of the tobacco leaves.