Abstract: The relative electric conductivity（REC）and field inspection were determined in leaves of six cultivars of Iris germanica during the natural winter along with dropping temperature. The logistic equation was constructed based on the relationship between REC and temperature，then the semi-lethal temperature was determined. Malondialdehyde（MDA）contents，soluble protein contents and free proline contents were analyzed. The results showed that the index of chilling injury was intervenient 1.20 and 3.65，and the range of semi-lethal temperature（LT₅₀）was from–13.81 ℃ to–5.73 ℃. ‘Cherry Falls’ and‘Bedtime Story’had high cold tolerance. ‘Brown Lasso’and‘Black Swan’had medium cold tolerance. ‘China Dragon’and‘Caligula’had poor cold tolerance. The LT₅₀ decreased with the drop in temperature， the decrease of LT₅₀ was cultivar dependent. The decrease of LT₅₀ varied from 1.46 ℃ to 6.51 ℃. The REC of the all cultivars increased following an S-curve as the temperature dropped. The contents of soluble protein，free proline and MDA increased firstly，then decreased.

Key words: Iris germanica, relative electric conductivity, semi-lethal temperature, cold injury index, cold tolerance, physiological response