Abstract: Some photosynthetic characteristics in Flue-cured tobacco were studied in a field experiment conducted during 1996～1997 with the application of fixed nitrogen levels and various proportions of different nitrogen forms. The results indicated that the content of chlorophyll in the leaves increased with an increase of proportion of ammonium-N (NH4+-N) during early and middle stages of growth, but descended with 100% NH4+ -N. In later stage of growth, the content of chlorophyll in leaves coincided with the ratio of NH4+ -N to NO3-N, it was the highest with 100% NH4+ -N. The activities of Hill reaction and photophosphorylation increased with increasing proportion of NH4+ -N, but the ratio of P/O remained almost the same. On characteristic fluorescence parameters and some ratios of the parameters, the values of Fm and Fv, the ratios of Fd/Fs and Fv/Fo as well as Fv/Fm increased with the increasing proportion of NH4+ -N, but the values and the ratios of treatment 4 (75% NH4+ -N + 25% NO3- -N) were the optimum.