Abstract: The defense mechanism of tumor immune response is triggered spontaneously with the onset of oncogenesis in hematological malignancy. However, the presence of activated immune cells and effector cytokines activates multiple immunosuppressive pathways prior to clinical diagnosis of tumors, which synergize with each other and cause dysfunction of tumor antigen-specific T cells, ultimately leading to a state of immune tolerance in hematological malignancies. Indoleamine-2, 3-dioxygenase (IDO) is an important member of these immunosuppressive pathways, which induces counter-regulation to limit the inflammatory response and triggers T cell-acquired tolerance, eventually inhibiting the tumor immune response. Considering the role of IDO in immunosuppression, IDO inhibitors constitute an important part of the immunotherapeutic arsenal against various tumors, especially hematological malignancies, and have been studied extensively in recent years. This review discusses the significance of IDO and its inhibitors in the treatment and prognosis of hematological malignancies.