Abstract: Using a chemistry transport model (SLIMCAT) and reanalysis data, this study investigates the extremely low ozone events in 1997 and 2011 in the Arctic stratosphere. The analysis reveals that, the magnitudes of total column ozone (TCO) anomalies over the Arctic in 1997 and 2011 both could be up to about -80 DU and the ozone decreases between 200 hPa and 30 hPa accounted for about 80% of the TCO anomalies. Our analysis suggests that the two extremely low Arctic TCO events were possibly related to La Nia activity, which resulted in a stronger Arctic polar vortex, a lower stratospheric temperature, more polar clouds, and eventually more ozone chemical loss. Furthermore, since the positive sea surface temperature anomalies in the North Pacific in 2011 led to a lower Aleutian low, a weaker troposphere wave forcing, a colder Arctic vortex and more type Ⅱ PSCs, the ozone chemical loss in the Arctic UTLS region was accelerated in 2011. A comparison of the ozone anomalies in different layers between 1997 and 2011 indicates that the ozone decrease in the Arctic UTLS region in 2011 was much larger than that in 1997.