Indoleamine 2,3-dioxygenase (IDO), an enzyme involved in the catabolism of tryptophan, is expressed by a variety of cells and tissues such as macrophages, dendritic cells, cells of the endocrine system and by the placenta. IFN- γ is the main inducer of this enzyme. IDO acts as an important defense mechanism of innate immunity against pathogens. It also has tumor suppressive activity and prolongs the survival of allograft. One of the interesting functions of IDO is prevention of the allogenic fetus rejection during pregnancy by inhibiting alloreactive T cells. It was shown that inhibition of IDO activity by IDO inhibitor, 1-methyl tryptophan, during mouse pregnancy causes fetal rejection. The main mechanism by which IDO protects fetus is through reducing the tryptophan level and suppressing the T cell activity in the feto-maternal interface. In this review the biological functions of IDO with emphasis on its role in allogeneic fetus protection have been discussed.