Abstract: The leaf litters of 10-year-old Quercus mongolica were put in nylon bags and exposed to elevated O₃  level (120 nmol·mol^-1) with the control of 40 nmol·mol-1 in open top chambers (OTCs) for 150 days to test the effect of high O₃ on the litter decomposition. The results showed that  no significant difference was observed in residual mass between elevated O₃ treatment and the control. Elevated O₃ inhibited the release of C and K during the decomposition, the residual rate of K under elevated O_3treatment (23.9%) was significantly higher than that of the control (17.1%) after 150day decomposition. Compared with the control, N mineralization and lignin degradation in elevated O₃ treatment were inhibited during early period of decomposition (0-60 d), but were promoted in later period (90-150 d). The changes of lignin/N showed no significant difference between elevated O₃ treatment and the control during the decomposition. Elevated O₃ generally promoted the release of P in leaf litter of Q. mongolica during the decomposition. C/P ratio was higher under elevated O₃ than that under control. Significant positive correlation was shown between residual dry mass of leaf litters and the residual rate of C, N, K, C/N ratio during decomposition. Elevated O₃ might play an important role in the nutrient cycle of forest ecosystem in highO₃ pollution area.