The prevalance of asthma and other allergic diseases have increased in recent years. Also, the changes of environmental factors are responsable in this increase. There are two hypothesis about the explanation to this increase, these are `hygen and vitamin hypothesis`. Several studies have shown the effects of Vitamin D on immun system especially T cells and antimicrobial peptides. All these studies demonstrated that Vitamin D might be responsable in development of allergic diseases by acting on T Helper cells especially Tregulatory cells. The purpose of this review is to present the evidence the role of Vitamin D in immun system and allergic diseases.