Elevated atmospheric CO₂affect the growth and physiological traits of forest trees. Many studies have revealed the changes in leaf quality such as increase in CN ratio and increase in defensive chemicals of trees grown under elevated CO₂. These changes might induce the changes in the relationship between plants and herbivores. In this study, the changes in phenology and traits of sun leaves of Siebold’s beech (Fagus crenata) grown at Free Air CO₂ Enrichment (FACE; ca. 500 ppm) system between 2003 and 2011 have been investigated. Here I report the results in 2011. The timing of leaf emergence of young beech trees at FACE was not changed compared to that at control site. Leaf mass per area (LMA) of beech leaves was highest at mid-June at both sites. Before mid-June, there was no significant deference in LMA between FACE and control. However, after mid-June, LMA of beech leaves at FACE was higher than those of control. Concentration of condensed tannin and total phenolics of leaves taken after mid-June at FACE were higher significantly than those at control. These results suggest that elevated CO₂ would induce changes in the leaf quality after mid-June after 9 years of CO₂ exposure.