Interferon-gamma (IFN-${gamma}$) has been used to treat various malignant tumors. However, the molecular mechanisms underlying the direct anti-proliferative activity of IFN-${gamma}$ are poorly understood. In the present study, we examined the in vitro antitumor activity of IFN-${gamma}$ on two human non-small-cell lung carcinoma (NSCLC) cell lines, H322M and H226. Our findings indicated that IFN-${gamma}$ treatment caused a time-dependent reduction in cell viability and induced apoptosis through a FADD-mediated caspase-8/tBid/mitochondria-dependent pathway in both cell lines. Notably, we also postulated that IFN-${gamma}$ increased indoleamine 2,3-dioxygenase (IDO) expression and enzymatic activity in H322M and H226 cells. In addition, inhibition of IDO activity by the IDO inhibitor 1-MT or tryptophan significantly reduced IFN-${gamma}$-induced apoptosis and death receptor 5 (DR5) expression, which suggests that IDO enzymatic activity plays an important role in the anti-NSCLC cancer effect of IFN-${gamma}$. These results provide new mechanistic insights into interferon-${gamma}$ antitumor activity and further support IFN-${gamma}$ as a potential therapeutic adjuvant for the treatment of NCSLC.