BACKGROUND: The pentavalent antimonial compounds are the first drug of choice for leishmania infection, but have several side effects that cause some restriction for use. Extension of nanoparticle use in biological research and proven effectiveness of manganese nanoparticles on fungi and bacteria, along with the lack of information about its antileishmanial effects, have motivated this study. Manganese can induce cell apoptosis by increasing FOXO3a-Bim/PUMA mRNA activation and activating of caspase-3 pathway. METHODS: This study was aimed to examine the efficacy of manganese oxide nanoparticles againstLeishmania major (MRHO/IR/75/ER) in vitro and in vivo. To evaluate the antileishmanial activity of NPs, light microscopic observation was used to determine the number of remaining parasites in each well. The MTT test was used to determine the cytotoxicity effects of MnO NPs against L. major promastigotes and macrophage cells. The effect of nanoparticles on cultured amastigotes under in vitro conditions was also investigated. The possible apoptosis of L. major by MnO NPs was evaluated with flow cytometry assay. Additionally, the preventive and therapeutic effects of MnO NPs in BALB/c mice following cutaneous L. major infection was tested. The effect of MnO NPs on promastigotes and amastigotes were proven by MTT assay and amastigote assay, respectively. RESULTS: The IC value of MnO NPs against L. major promastigotes and macrophages was 15 and 40 μg ml respectively. The results of flow cytometry showed about 57% of the promastigotes were induced to apoptosis with MnO NPs. In in vivo studies, the size of the ulcers were significantly reduced, and the survival rate of the mice, in comparison with the control group, was increased. CONCLUSION: MnO NPs has a beneficial effect on L. major promastigotes in vitro and in vivo and could be considered as a candidate for the treatment of this infection.