The effects of P and K fertilizers on the carbon and nitrogen metabolism and photosynthetic efficiency of dark sun-cured tobacco were studied with a pot experiment. The results showed that with equal N rate (150 kg N per hectare), the nitrate reductase (NR) activity, invertase (INV) activity, and chlorophyll contents of fresh leaves decreased without P or K fertilizer application. The NR/INV ratio was higher in case of no or low P application and the values of Fv/Fo and Fv/Fm decreased in either case of no or low P and K application. The effect of P deficiency was severer than K deficiency before fast growing stage. The application of N 150 kg/hm2 with N:P2O5:K2O=1:2:2.5 was in favor of the balance of carbon and nitrogen metabolism.