To investigate the responses of yellow poplar (Liriodendron tulipifera L.) seedlings to the interactive effects of the elevated atmospheric CO₂ level and nitrogen addition, we measured biomass, photosynthetic pigments, photosynthesis, and the comtents of nitrogen (N) and carbon (C) from the seedlings after 16 weeks of the treatments. Yellow poplar seedlings were grown under the ambient (400umol mol?¹)and the elevated (560 and 720 mol?¹) CO₂concentratoins with three different N addition levels (1.2, 2.4, and 3.6 g kg?¹) in the Open Top Chambers (OTC). The dry weight of the seedlings enhanced with the increased N levels under the elevated CO₂concentrations and the increment of the dry weight differed among the different N levels. Photosynthetic pigment content of the yellow poplar leaves also increased with the increase of the CO₂concentration levels. The effects of the N levels in the photosynthetic pigment cont, were significantly different among the CO₂levels. Photosynthetic rates were affected by the levels of CO₂and N concentrations. Stomatal conducatance and transpiration rates increased with increasing CO₂concentration. The carboxylation efficiency of the seedlings without N addition increased under the higher CO₂concentrations whereas that with N addition decreased under the elevated CO₂concentrations. Nitrogen and carbon uptake in leaf, stem, and root increased with the elevated CO₂concentration level and N addition. In conclusion, under the elevated CO₂concentrations, physiological characteristics and carbon uptake of the yellow poplar seedling were improved and increased with N addition.