Abstract: The combined oral contraceptives（COCs） are the main methods with steroidal hormones for birth control at present. For half a century，the more safe，reliable and effective COCs were continuously developed. The chemical composition of estrogen in COCs have also been improved. This article summarized the early process of modern COCs and the development of estrogen，as well as the contraceptive mechanism of ethinylestradiol and mestranol. The two developing areas of estrogen in COCs are as follows：to reduce its dose to achieve an optimal concentration for the body′s sensitivity and compliance，to find some better estrogens than ethinyl estradiol.

Key words: Contraceptives, oral, combined, Estrogens, Ethinyl estradiol, Mestranol, Levonorgestrel