Inclusion Criteria:

1. female volunteers of childbearing potential;
2. are first time users of OC or have discontinued OC at least 2 months prior to study entry;
3. age between 18 and 35 years old;
4. normal body mass index (18-30);
5. has signed consent form; and
6. is in good health as confirmed by medical history, physical examination

Exclusion Criteria:

1. a positive pregnancy test will automatically exclude the volunteer from participation in this study.
2. any contraindication for oral contraception use;
3. known hypersensitivity to Letrozole and co-administered medications;
4. irregular menstrual cycles;
5. ultrasonographic evidence of ovarian dysfunction, such as Polycystic Ovary Syndrome (PCOS);
6. history of pituitary tumor;
7. HIV, HBV, HCV infection;
8. vaginal infection;
9. abnormal ECG;
10. abnormal lab tests for blood profile, liver function and renal function;
11. uncontrolled diabetes and blood pressure;
12. pregnancy (suspected or diagnosed) or lactation;
13. history or suspicion of drug or alcohol abuse;
14. history of severe mental disorders;
15. participation in an investigational drug trial within the 30 days prior to selection;

16. exhibits a disorder that is a contraindication to steroid hormonal therapy, including, for example, the following conditions:

    • history of, or actual, thrombophlebitis or thromboembolic disorders.
    • history of, or actual, cerebrovascular disorders.
    • history of, or actual, myocardial infarction or coronary artery disease.
    • acute liver disease.
    • history of, or actual, benign or malignant liver tumors.
    • history of, or suspected, carcinoma of the breast.
    • known, or suspected, estrogen-dependent neoplasia.
    • undiagnosed abnormal vaginal bleeding.
    • any ocular lesion arising from ophthalmic vascular disease, such as partial or complete loss of vision or defect in visual field.