Abstract: Indoleamine 2, 3-dioxygenase 1(IDO1)is the rate-limiting enzyme which catalyses the metabolism of L-tryptophan(L-Trp)in the kynurenine pathway. It is overexpressed in many tumor cells and antigen presenting cells. This enzyme inhibits local immune response and supports tumor cells to evade immune surveillance by depleting L-Trp and producing kynurenine metabolites, thus, it is an important target for cancer immunotherapy. There are several IDO1 inhibitors with different scarfold under investigation, three of which have already entered clinical stage. The role of IDO1 in tumor immune tolerance and the research progress on IDO1 inhibitors in recent years are summarized in this paper.