Objective: Study on the pharmacology function of treating the breast cancer with aromatase inhibitors. Methods: Using outside organize experiments to make sure that formestane is operative inhibitor, we study its pharmacological function through the clinical experimental observation. Results: Formestane was the comparatively effective inhibitor of CYP19 in the organism get 85~90% of the restrain and emergence qualitative change. Conclusion: There are better results in the treatment of the breast cancer with CYP19 inhibitor formestane.