Abstract: The single or interactive effects of double CO₂ and O₃ concent rations on soybean (Gly cine max L.Merr.) are investigated for first time by using a soybean cultivars, Zhonghuang-14, in an open-top chamber.Cont rasting to the control, the effects of doubled CO₂ concentration alone on soybean biomass, yield, legumen bunches, legumen numbers, grain numbers and weight are posit ive, but the effects of doubled O₃ concent ration alone on the above parameters are obviously negative; and the longer the ventilation time periods are, the more obvious the effects are.When both CO₂ and O₃ concentrations are doubled, their interaction shows that the effect of CO₂ is greater than that of O₃.In the process of CO₂ and O₃ concentrations increasing gradually, the negative O₃ effect approached the posi tive CO₂ effect eventually because of the gradually cumulating O₃ concent ration and gradually increasing stimulation.The doubled O₃ concent ration alone suppresses photosynthetic products into the soybean roots and grains, but increases those into soybean stems and leaves, resulting in the reduced root/shoot ratio (RSR) and grain/culm ratio (GCR).The long-period effect reduces Harvest Index (HI) and increases Leaf Weight Ratio (LWR) markedly.With the ventilation time prolonged the effects are more remarkable.The interactive effects of doubled CO₂ and O₃ concentrations on the RSR、LWR、GSR and HI are relatively slight, only within±10%.