Letrozole is a new generation of highly selective aromatase inhibitor, which is a synthetic benzyltriazole derivative. By inhibiting aromatase, letrozole can reduce the level of estrogen, so as to eliminate the stimulating effect of estrogen on tumor growth. The in vivo activity is 150-250 times stronger than that of the first generation aromatase inhibitor amlumide. Because of its high selectivity, it does not affect the function of glucocorticoid, mineralocorticoid and thyroid. High dose use has no inhibitory effect on the secretion of adrenal corticosteroids, so it has a high therapeutic index. Preclinical studies have shown that letrozole has no potential toxicity, mutagenic and carcinogenic effects on various systems and target organs of the body, and has less toxic and side effects, good tolerance. Compared with other aromatase inhibitors and anti estrogen drugs, letrozole has stronger anti-tumor effect. It is suitable for the treatment of patients with advanced breast cancer who are not effective in the treatment of estrogen and early treatment of breast cancer.