To investigate the efficacy and safety of eldecalcitol in postmenopausal women with low bone mineral density (BMD) or mild osteoporosis through a randomized, open-label, parallel controlled trial with calcitriol as the control.

Postmenopausal women with low bone mineral density (BMD) or mild osteoporosis will be randomized in a 1:1 ratio to receive open-label oral 0.75μg eldecalcit daily or open-label oral 0.5μg calcitriol daily for 12 months.

Inclusion Criteria:

1. Postmenopausal women (based on medical history) aged 50 or older and menopause time more than 2 years. If there is uncertainty regarding menopausal status, women 60 years of age and older will be considered postmenopausal.
2. BMD (determined by DXA) of lumbar L1-4 or total hip or femoral neck met one of the following conditions at screening:

1) low BMD: -2.5<T< -1.0, without vertebral and fragility fracture history 2) mild osteoporosis: T≤-2.5, without vertebral and fragility fracture history 3. Ambulatory outpatients 4. Written informed consent

Exclusion Criteria:

1. Severe vitamin D deficiency at screening (defined as serum 25OHD<10 ng/ml);Vitamin D repletion will be permitted and dosage will be at the discretion of investigators and subjects may be re-screened
2. Primary hyperparathyroidism, Cushing's syndrome, hyperthyroidism, gonadal insufficiency, osteogenesis impotence, rheumatoid arthritis, poorly controlled diabetes mellitus (HbA1c>9%), or other conditions that can cause secondary osteoporosis, or treatment with glucocorticoids in the 2 months prior to study entry
3. Had injected bisphosphonate once or more within 3 years before entering the study; or had received oral bisphosphonate oral for 6 months or more in the 3 years prior to study entry; or had received parathyroid hormone once or more in the 3 years before entering the study; or received 2 weeks or more of calcitonin or vitamin K or active vitamin D or selective estrogen receptor modulator or hormone replacement therapy or traditional Chinese medicine for bone pine within 2 months prior to study entry; or received subcutaneous anti-RANKL antibody at any time prior to study entry
4. Urolithiasis at screening
5. Hypercalcemia (serum calcium >10.4 mg/dL), or hypercalciuria (urine calcium>400 mg/gCr), or hyperuricemia (above 420 μmol/L in men and 360 μmol/L in women)
6. Severe liver disease such as cirrhosis or serious heart disease such as severe heart failure
7. Active malignant tumors or a history of malignant tumors within 5 years before informed consent obtained
8. History of allergy to vitamin D
9. Any condition which in the opinion of the investigator unfit for the study