Abstract: The effects of nitrogen rates on the physiological characteristics (including chlorophyll fluorescence) of dark sun-cured tobacco were studied with pot experiments. The results showed that with the increase of N rates, the NR activity, INV activity and chlorophyll content of fresh leaves were enhanced gradually, however, the NR activity decreased in case of high N application and the NR/INV ratio was lower in either case of no or high nitrogen application at rosette stage. The values of Fv/Fo and Fv/Fm increased with the increase of N rates in a certain range, while the values decreased when the N rate was over 150kg/hm² before fast growing stage.