Indoleamine 2,3-dioxigenase 1 (IDO1) acts in pathogenic inflammatory processes and engender immune tolerance to tumor antigens. IDO1 can decrease the tryptophan and produce a series of toxic kynurenine metabolites to promote the immune toleration via GCN2 pathway, mTOR pathway, toxic effect of kynurenine and favoring differentiation of Tregs. IDO1 can be induced in most human cells, especially APCs and cancer cells through canonical and non-canonical NF-kappaB and Jak/STAT pathways, as well as PKC and TGF-beta signaling pathways. A series of human cancers over-express IDO1 in a constitutive way. Thus, IDO1 is likely to be an attractive target for developing inhibitors of tumor treatments. Many preclinical and clinical trials have been underway and suggest that IDO1 inhibitor maybe an effective tool against a wide range of cancers. However, the IDO1 inhibitor alone had been verified that to be disappointment in achieving effective antitumor efficacy. Concentrating on its molecular mechanism in immune toleration and complex environments of cancer, IDO1 inhibitor could cooperate with chemotherapies and other immune target inhibitors to lessen the tumor.