Leishmaniosis is a parasitic infection spreads to humans by sand flies. Over 20 different species of Leishmania are responsible for the disease, which infect over 14 million people around the world. Chemotherapy is one of the most effective treatments for leishmaniosis, however it is restricted by the high cost and/or toxicity. In this study, the possible effect of artemisinin (ART) was detected on intracellular amastigotes of Iraqi strain of Leishmania donovani in ex vivo condition in U937 macrophage cell line. Gene expression of inducible nitric oxide synthase (iNOS) in U937 macrophage was investigated, before and after treatment with artemisinin. Kinetic result by real-time PCR demonstrated that the iNOS expression folding reached the maximum at concentration of 500 μM after 24 hours, at 750 μM after 48 hours and at 1000 μM after 72 hours, which was 56, 11, and 6, respectively. The copy number of iNOS gene expression was also significantly higher in treated infected U937 cells compared to both non-treated-infected cells and intact macrophages, under different concentration of ART along the three times of follow-up. Moreover, stained macrophages with fluorescent DAPI proved that the percentage of intracellular infective amastigotes was decreased to the minimum in treated U937 cells, in comparison to non-treated cells. The minimal amastigote-invasion percentage was recorded at 1000 μM, which was 26%, 27%, 21% compared to 61%, 87%, 75% in untreated cells after 24, 48, 72 hours respectively. These findings demonstrated ART positive efficacy against iNOS expression and this compound can be further studied as novel therapeutic rather than toxic available chemotherapies.