Indoleamine 2, 3-dioxygenase (IDO) as an intracellular cytosolic enzyme converts tryptophan (Trp) to -formyl kynurenine which leads to proinflammatory T-cell apoptosis and prevention of immune cells maturation via decreasing the level of cellular energy. Trp catabolism products such as kynurenine increase the recruitment of regulatory T cells and induce immune tolerance in dendritic cells. IDO expression can locally suppress immunity in the tumor microenvironment and tumor progression actively recruits IDO expressing cells in tumor-draining lymph nodes. Also, tumor infiltrating Tregs' activity leads to IDO expression in the tumor microenvironment. In this review, we described the immunomodulatory function of IDO and IDO-based therapeutic strategies for immune related diseases. According to positive-feedback loop between Tregs and IDO in the tumor microenvironment, IDO can be targeted as a promising immunostimulatory approach for immunotherapy of cancer. However, several studies revealed controversial consequences for influences of IDO in immunity. Considering the common concept, IDO1 and also IDO2 repress the function of T lymphocytes, while inactivation of IDO results in aggravation of some autoimmune diseases. Eventually, the extensive evaluation of IDO function in immunomodulatory procedure can help achieve IDO inhibitors as optimal drugs to inhibit tumor growth without motivating autoimmunity.