Through two consecutive years evaluation of lettuce grown in summer greenhouse, the diversity of 28 traits of 253 lettuce materials introduced from United States Department of Agriculture, World Vegetable Center, and Shanghai Agrobiological Gene Center was analyzed.The result showed that leaf attitude, leaf lobes, undulation of leaf margin, showiness of leaf pustules, leaf color, leaf shape and some other traits had highly genetic diversity. The coefficient of variation of the 28 character ranged from 0.0315 (glossiness of leaf) to 2.7366 (intensity of leaf anthocyanin coloration), with the mean value of 0.7444. The diversity index of 28 characters ranged from 0.0258 (glossiness of leaf) to 1.9700 (fresh weight of aerial parts). According to the cluster analysis, the 253 materials were divided into 6 groups, with group I containing 99 materials, group II 38 materials, group III 84 materials, group IV 5 materials, group V 9 materials, and group VI 18 materials. In this study, the 253 materials were synthetically evaluated in the field under high temperature in summer and 14 excellent materials with strong heat tolerance and late bolting were preliminarily screened based on the investigation of heat damage and bolting time. The identification and evaluation of heat tolerance of lettuce germplasm resources provide variety materials for resource innovation, gene cloning, and mechanism research for lettuce heat tolerance in the future.