【Background】Transgenic Bt cotton has been grown commercially in a large scale in China for nearly 20 years. The evolution of resistance by the target pest, the cotton bollworm (Helicoverpa armigera Hübner) is the most important threat to the sustainable development of the cotton industry. Resistance monitoring is a necessary management measure to extend the usefulness of transgenic Bt cotton. The Yancheng area is the principal planting area of transgenic Bt cotton in Jiangsu Province, but the resistant gene frequency of H.armigera in this area has not been reported. 【Method】Populations of H.armigera were collected in the cotton fields near Yancheng City in 2012 and the sensitivity of newly hatched larvae to the young leaves of Bt cotton (Zhong-30) was tested. The resistance allele frequency of second instar larvae was studied using the distinguishing dosage method. 【Result】All the individuals of newly hatched larvae of H.armigera from Yancheng died within 9 d after feeding on transgenic Bt cotton leaves. The resistance allele frequency to Bt protein of second instar larvae from Sanlong and Dongtai Towns were 7.6×10-3 and 6.9×10-3 respectively. 【Conclusion and significance】The current populations of H.armigera from the Yancheng cotton fields remained highly sensitive to transgenic Bt cotton, and their frequency of resistance alleles did not change significantly. However, continuous monitoring of the resistance allele frequency in field populations is still necessary.