A simple,cheap,and efficient technique for transgenic Bt cotton using kanamycin as an indirect identification marker in the laborotary is tested.The result showed that kanamycin could be used in the laborotary to test whether or not a plant is resistant.Seed of transgenic Bt cotton were cultured on sand bed containing 4000mg/L kanamycin solution for six days.The cotyledons of resistant varieties remained normal green color,whilst these of the commercial varieties all turned yellow.This technique is especially usful for the purity test of transgenic Bt cotton before the seed is sold. Meanwhile,the hypocotyls and fresh weigh of the cotton seedling were affected by the kanamycin in some degree.