The purpose of the present study is to investigate the potential effect of vitamin K on the Bone mineral density of patients with postmenopausal osteoporosis. This open, randomized, parallel study will be conducted on patients with osteoporosis according to WHO Criteria (T≤ -2.5) 2) who will attend to the rheumatology clinic of Imam Reza Hospital, Mashhad, Iran. Eligible patients are those who meet all the following criteria: 1) Osteoporosis according to WHO Criteria (T≤ -2.5) 2) Age: 45-65 years 3) postmenopausal women. Patients are not eligible if they have: 1) secondary osteoporosis 2) Taking anticoagulants 3) History of liver and kidney disorders 4) History of bisphosphonates side effects 5) People who have contraindications to take calcium and vitamin D 6) People who take other anti- osteoporotic medications and 7) People who needs to use Vitamin K in the group 1. The total number of patients will be fifty cases. The patients will randomly assign into two groups: group 1 (n=25) will receive calcium + vitamin D pills twice daily (Jalinous company, Tehran, Iran, each pill has 500 mg calcium and 200 IU vitamin D), 70 mg Alendronate orally per week (Alenate®; Dr. Abidi Company, Tehran, Iran). Group 2 (n=25) will receive the same combination plus vitamin K1 pills 10 mg daily (Pars Minoo Ind. Company, Tehran, Iran). The duration of treatment will be one year in both groups. The main outcome measure will be bone mineral density. At the end of the study, we will measure bone mineral density again for all of the patients by the same scanner.