Vitamin D plays an important role in calcium homeostasis and metabolism. Vitamin D3 and vitamin D2 are two forms of vitamin D in human body, both of which are contained in only a few foods. The deficiency of vitamin D in human body can cause a threat of diseases such as osteomalacia, rickets and diabetes. 25-OH Vitamin D2 and 25-OH Vitamin D3 as vitamin D metabolites are measured in human serum to evaluate vitamin D level. Vitamin D supplements are considered as an effective way to treat or to prevent the diseases.