Several lives of evidence indicate a protective effect of vitamin K against osteoporosis. Epidemiological studies showed that low vitamin K intake is associated with the increased risk of osteoporosis. Vitamin K2 (menatetrenone, MK-4) has been clinically used in the treatment of patients with osteoporosis in Japan, Korea and Thailand. Previous studies demonstrated the efficacy of vitamin K2 (45 mg/day) to prevent bone loss and reduce the rate of vertebral fractures, although a large, randomized intervention study is anticipated to provide more detailed evidence. Recently, vitamin K2 has been shown to reduce the progression of hepatocarcinoma. Moreover, it has been proposed that vitamin K may also have beneficial effects to prevent atherogenesis. The clarification of molecular mechanisms by which vitamin K2 exerts these salutary effects deserve further investigations.