Four F1 hybrids were obtained between Selected – PSU (yardlong bean) as a susceptible cultivar and four cultivars of cowpea that had been reported as resistant cultivars; IT82E – 16, SR00 – 863, Suranaree 1, and Khao – hinson were planted in replicated plots under screenhouse conditions at the Department of Plant Science, Faculty of Technology and Community Development, Thaksin University, Phatthalung Campus Phatthalung Province, Thailand, between September and November 2009. The experimental design was a Randomized Complete Block Design (RCBD) with four replications (each replication consists of ten plots). Five apterous adult aphids were released on each plant at three weeks after planting. A number of aphids and visual scored damages were determined for each generation. Moreover, collected samples compared with stem structures. The results showed that the highest number of aphids infestation were found on Selected – PSU cultivar about 6535.02±993.56 aphids/plant. The lowest number of aphids was recorded on IT82E – 16 cultivar about 1606.81±215.34 aphids/plant. The same results were obtained from visual scored damages. When comparing cell structure, Selected – PSU cultivar gave the thinnest epidermis and collenchyma. Whereas, IT82E – 16 cultivar gave the thickest epidermis and collenchyma.