Abstract: We analyzed the anticancer effect and mechanism of the novel indoleamine 2,‍3-dioxygenase 1 (IDO1) inhibitor NLG-919 combined with temozolomide (TMZ) on human glioma cell lines. The anti-tumor activity of NLG-919 and temozolomide after single and combined treatments was detected by MTT assay. Colony formation assay, invasion assay and migration assays were used to detect the effects of NLG-919 and temozolomide alone or in combination on proliferation, invasion and migration of human glioma cells. A flow cytometry assay was used to detect cell apoptosis, cell cycle arrest, reactive oxygen species (ROS) production and mitochondrial membrane potential damage (JC-1). An immunofluorescence assay was used to detect the expression level of IDO1 and HPLC was used to detect the expression level of L-kynurenine (Kyn) to explore the anti-tumor mechanism of NLG-919 and temozolomide. The results show that NLG-919 had a weak in vitro inhibitory effect compared to that of temozolomide. The IC₅₀ of NLG-919 on U251 cells and U87 after 72 h was 26.9 and 30.7 μmol·L^-1, respectively. However, when NLG-919 was used in combination with temozolomide, its anti-glioma activity was significantly increased. Compared with the single treatment, the combination treatment had a potent ability to inhibit proliferation, invasion and migration of glioma cells. Combination treatment improved the capacity of temozolomide to induce cell cycle arrest and inhibit the growth of glioma cells. NLG-919 significantly down-regulated the expression and activity of IDO1 in glioma cells, and the inhibitory effect was improved after combination with temozolomide, and effectively blocked the production of Kyn through the metabolism of L-tryptophan (Trp). In conclusion, the IDO1 inhibitor NLG-919 and temozolomide showed synergistic effects in the anticancer therapy of human glioma cell lines.