Abstract: The study on the response of a mutant and a wild type of Arabidopsis to 660 μl·L^-1 CO₂ and ambient CO₂ showed that under elevated CO₂,the stomatal numbers of the mutant increased,while those of the wild type decreased. The chlorophyll content and NR (nitrate reductase) activity of the mutant increased,but those of the wild type had no obvious response. The mutant was not reproductively mature after the continuous exposure to increased CO₂ for five months. The results provided evidence of plant response to the changes of atmospheric CO₂ concentration,and the clues to related studies on other plants.