Abstract: Indoleamine 2,3-dioxygenase 1 (IDO1) is a key enzyme of L-tryptophan metabolic oxidation pathway, in which the L-tryptophan is transformed into N-formyl kynurenine by oxidative cleavage. IDO1 is considered as a potential target for the development of cancer immunotherapeutic molecules. Up to now, at least 10 drug candidates have been advanced into clinical research. In this review, the binding mode and structure-activity relationships of the representative IDO1 small molecule inhibitors were summarized according the characteristics of chemical structures. Hopefully, this review could provide some insights for further development of novel IDO1 inhibitors.