The effect of Pinus sylvestriformis seedlings density on net photosynthetic rate was studied under elevated CO2. Atmospheric CO2 concentration was controlled in OTC (Open Top Chamber). The results showed that elevated CO2 not only made net photosynthetic rates (NPRs) of two Pinus sylvestriformis seedlings densities increased, but also mitigated their intra-specific competition. Meanwhile, the difference of seedling NPRs between 100 and 400 plant.m-2 under 500 mumol.mol-1 air CO2 concentration was less tha that under 350 mumol.mol-1 with the same PAR levels. When air CO2 concentration reached 700 mumol.mol-1, the NPRs of seedlings under both planting densities were close to each other with the same PAR levels. The intra-specific competition was minimized under air CO2 concentration of 700 mumol.mol-1.