Abstract: To detect the adaptability difference under high temperature of different lettuce cultivars， leaf segments of seven lettuce cultivars （A\|G） were used to study the heat tolerance by electrical conductivity method and antioxidant enzyme indexes. The results showed that with the rise of treated temperature， cells injured rate of leaf segments from A to G were increased with trends of slow\|fast\|slow， showed typical S\|type curve， the significance test of which was fit Logistic equation. The heat resistance of the seven lettuce cultivars followed the order: C>D>F>B>G>E>A， respectively. Meanwhile， treating with high temperature for 6 d increased the superoxidase （SOD）， peroxidase （POD）， catalase （CAT） activities and the malondisldehyde （MDA） contents of various lettuce cultivars. The relationship between semi\|lethal high temperature and antioxidant enzyme indexes was significant. The above results showed that semi\|lethal high temperature could be considered as the index of lettuce heat resistance.

Key words: lettuce, semi\, lethal high temperature, heat tolerance, Logistic equation, antioxidant system