Letrozole is part of a new generation of highly selective aromatase inhibitors and is an artificially synthesized benzotriazole derivative. Letrozole inhibits aromatase to lower estrogen levels, thus preventing estrogen from stimulating tumor growth. Its in vivo activity is 150-250 times stronger than that of first generation aromatase inhibitor Amarante. As it is highly selective, it will not impact glucocorticoid, mineralocorticoid and thyroid functions; even at high dosages, it will not have any inhibiting effects on adrenal corticosteroid secretion, giving it a high treatment index. Letrozole has no latent toxicity towards any bodily systems and target organs, has no mutagenicity and carcinogenic effects, has minimal toxic side effects, is well-tolerated, and has stronger anticancer effects than other aromatase inhibitors and antiestrogen drugs. Letrozole is suitable for advanced breast cancer postmenopausal patients who have not responded to estrogen-suppressing treatment and for early breast cancer treatment. It is used to treat postmenopausal patients with advanced breast cancer and serves as a second-line treatment to follow unsuccessful antiestrogen treatment. Compared to the current standard Tamoxifen treatment, Letrozole can better prevent the risk of breast cancer recurrence.