The atmospheric CO₂ concentration is now increasing and will reach 700 ppm at the end of this century. Relative growth rate (RGR), an important component of fit ness, is an integration of various physiological and morphological traits. Here we questioned (1) whether within-species variation exsits in CO₂ response of RGR and, if so, (2) what physiological or morphological traits are responsible for the variation. We thus studied among-genotype variation in RGR at two CO₂ concentrations in Arabidopsis thaliana. There was a considerable variation in RGR among genotypes, which was explained mainly by a physiological trait, net assimilation rate (NAR), rather than by morphological traits such as leaf area ratio. The genotypes with higher RGR at the high CO₂ tended to have a greater response to CO₂ enrichment, while RGR at ambient condition was less related to the RGR response. We determined carbon uptake (photosynthesis) and loss (respiration) rates of the genotypes. NAR was positively correlated with the carbon balance and was explained mainly by photosynthesis. Carbon loss at the high CO₂ concentration was negatively correlated with carbon balance, which suggests that lower respiration rate also partly contributed to increasing carbon gain of the plants.