Abstract: An experiment of hydroponic culture of rice was carried out with treatments different in nitrogen supply and CO₂ level on the FACE(free air carbon-dioxide enrichment) platform to investigate growth,C/N ratio and allelochemical contents of rice plants of different allelopathic varieties.Interactions between allelochemicals and C/N ratio in rice plants were also analyzed.Results show that the growth of both varieties was enhanced significantly.In Treatments LN(lower nitrogen level) the C/N ratio in the plants increased in elevated CO₂,while in Treatments HN(high nitrogen level) it did not alter much.The total content of secondary metabolic material and characteristic allelochemicals of the rice plants of allelopathic variety also increased significantly,but such increment did not occur in rice plants of normal variety.C/N ratio of rice of allelopathic variety was positively correlated with contents of secondary metabolic materials and of characteristic allelochemicals as well.