Abstract: A case study on the ozone downward transport from stratosphere to troposphere over Xining (36.43°N, 101.45°E, 2296 m asl) on August 1st, 1996 has been given out by the methods of observational analysis, 3-dimensional backward trajectory calculation and the mesoscale modeling. Routine radiosonde data show that the main synoptic characteristics of this case were the east erly shift of the trough in the upper troposphere. As a result of the synoptics, a pronounced ozone peak was detected around 7 km over Xining. 3-dimensional trajectory calculations imply that the air parcels of ozone peak around 7 km may be traced to the middle Asia region, but the distinguished downward transport from strat osphere to troposphere induced was over the region from Xinjiang to Qinghai. Furthermore, mesoscale simulation confirms that the temporal and spatial evolution of tropopause folding indexed by the potential vorticity is the critical mechanism of such downward incursion. The modeling work also shows that the altitude of tropospheric ozone peak was associated with stable quas-izero divergence layer, which reflects the temporal dynamical balance in troposphere.