Vitamin D plays important role in calcium homeostasis and bone metabolism. However, the researches performed during the past twenty years revealed that it also has a significant immunomodulatory effect. D vitamin deficiency has been shown to be related to increased tendency for development of several autoimmune diseases such as type 1 diabetes and inflammatory bowel diseases. D vitamin deficiency has also been associated with increased risk of development of infections such as tuberculosis and malignancy. In experimental models, it has been found to be successful in reducing the risk of graft rejection. D vitamin has been found to have salutary effects on renal allograft function. The findings of the researches indicate that vitamin D might have a role as an alternative and safer treatment modality in many autoimmune diaseases and transplantation practice.