Abstract: Under FACE（free-air carbon dioxide enrichment）condition,root growth was monitored of rice（C₃ crop）and barnyardgrass（C₄ weed）in hydroponics with two levels of nitrogen supply.Results show that root biomass,root volume,root length and root diameter of rice and barnyardgrass all increased significantly under FACE condition with NN（normal nitrogen）supply,the response of rice to CO₂ was higher than that of barnyardgrass.FACE condition with LN（low nitrogen）supply did not significantly promote rice growth because of N stress,but response of barnyardgrass to CO₂ was not limited by N stress,showing significant increase in growth.N concentration of the root decreased significantly in elevated CO₂ but C concentration did not change remarkably,which led to obvious increase in ratio of C/N.Elevated CO₂ also decreased remarkably the number of root hair and per-unit dry root weight of rice and barnyardgrass,which might be one of morphological reasons why root activity decreased under FACE condition.