Abstract: The growth and competition of a C₃ crop rice and a C₄ weed, barnyardgrass under FACE(free-air carbon dioxide enrichment) conditions were studied by field experiment. The results showed that under FACEcondition,the biomass and yield,leaf number, tiller number,and leaf area index of rice increased, while those of barnyardgrass decreased.Both of the leaf area indexed of rice and barnyardgrass were decreased, while their net assimilation rate increased. When the ratio of planting density of rice and barnyardgrass was 1:1, all of the ratio of biomass, yield, leaf area index, tiller and net assimilation rate were increased under FACEcondition. It indicated that the competition between rice and barnyardgrass changed under FACEcondition. The competition ability of rice(C₃ crop) was enhanced relatively, while that of barnyardgrass weakened.