Abstract: The biomasses of root system, stem and leaves, the photosynthesis, the carbon (C) and nitrogen (N) accumulation and distribution of Populus davidiana × P. tremuloides seedlings under 4 different N and 4 phosphorus (P) nutrition levels were studied by sand culture in greenhouse. The results showed that the dry biomasses of different organs, the chlorophyll content, the photosynthesis rate and C and N accumulation were all influenced by N levels. The aboveground biomass was the highest under middle level of N nutrition supply ( 8 mmol/L), while the biomass of root system was the highest under lower N level (4 mmol/L). Both of the highest total biomass and the highest aboveground biomass were obtained at the intermediate P level (1.000 mmol/L). The contents of chlorophyll a, chlorophyll b, chlorophyll (a+b) and carotenoid content all increased along with the N concentration varied from the lowest level (1 mmol/L) to the highest level (16 mmol/L). But the four indexes were not significantly correlated with different P concentrations. In general, higher N level was more beneficial to the C and N accumulation of root system. But the C and N accumulation in the stem and leaves reached the highest at the intermediate N level. The different P level did not affect much on the C and N accumulation in the seedlings.