High temperature stress on different varieties of lettuce seedlings in MDA, proline and soluble sugar content were studied. The results were as follows : With the stress temperature, varieties of leaf lettuce seedlings in the MDA, proline and soluble sugar content gradually increased; Different varieties had the most significant difference in 38/33℃(d/n). The increase of heat-resistant varieties on proline and soluble sugar content was quick and high, while its increase in MDA content was slow and small.