Abstract: In order to investigate the quantitative relationships between nitrogen fertilizer use efficiency and the contents of alkaloid components (mass fraction) in flue-cured tobacco, four nitrogen application treatments (140.81 kg/hm² (T1), 156.45 kg/hm² (T2), 172.10 kg/hm² (T3), and 187.74 kg/hm² (T4)) were applied using Yueyan 97 as the experimental flue-cured tobacco. The mass fractions of nitrogen content and alkaloid components (total alkaloids, nicotine, nornicotine, neonicotine and anatabine) in the upper, middle and lower leaves, roots and stems of the flue-cured tobacco at rosette, fast growing and maturity stages were determined. ¹⁵N isotopic tracing was used to analyze ¹⁵N distribution rate in different parts of tobacco plants. Nitrogen accumulation, nitrogen fertilizer use efficiency by the plant and nitrogen fertilizer contribution rate were calculated, and the quantitative relationships between the nitrogen fertilizer use efficiency and the contents of alkaloid components were analyzed. The results showed that the contents of alkaloid components in different parts of the plant increased with the increase of nitrogen application rate at the rosette and fast growing stages. At the maturity stage, the nitrogen fertilizer use efficiency under the T3 treatment was the highest, which was 111.06% higher than that under the T1 treatment, and the nitrogen accumulations in the upper, middle and lower leaves as well as ¹⁵N distribution rate in the middle leaves under the T3 treatment were the highest, which were 56.34%, 80.82%, 58.62% and 29.03% higher than those under the T1 treatment. The nicotine contents in the upper, middle and lower leaves as well as ¹⁵N distribution rate in the upper leaves under the T4 treatment were the highest, which were 59.26%, 62.75%, 72.34% and 21.56% higher than those under the T1 treatment. Fitting analysis between the nitrogen fertilizer use efficiency and the contents of alkaloid components revealed that, with the increase of nitrogen application rate, the nitrogen accumulation, nitrogen fertilizer use efficiency and the contents of alkaloid components at the rosette and fast growing stages increased, indicating that the increase of nitrogen application rate was conductive to the accumulations of the alkaloid components at the earlier stage of growth and development period of the plant. At the maturity stage, the fitting curve between the nitrogen fertilizer use efficiency and the contents of alkaloid components (except nornicotine) were parabolic, and the nitrogen application rate exceeding the threshold value would pose an inhibitory effect on the accumulations of alkaloid components in the plant. When the nitrogen fertilizer use efficiency of tobacco plants was 28.02%, the nicotine content in tobacco plant was the highest.