Background & Aims: Plant extracts and plant-derived compounds are valuable sources of new medicinal agents that are commonly used to treat infectious diseases. The purpose of this study was to evaluate, antiLeishmanial activity of three plant extracts on L.major promastigotes by colorimetric assay as compared to a trivalent antimonial compound (tartar emetic). Methods: Leishmania major promastigotes were cultured at 25±2°C in stationary phase in RPMI-1640 medium supplemented with 10% heat inactivated fetal calf serum (FCS) and antibiotic. Then by using MTT assay, the biological activity of plant extracts in comparison to potassium antimonyl tartrate [Sb (III)] on L.major promastigotes were assessed. The optical density (OD) due to cleavage of the tetrazolium salt MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide] into a colored product formazan by the parasite was measured using ELISA reader and IC50 values (50% inhibitory concentrations) were determined. All experiments were repeated in duplicate. Results: plant extracts and Tartar emetic inhibited the growth of promastigote forms of L.major in vitro after 72 hours of incubation. IC50 of tartar emetic was 4.7 µg/ml, and IC50 values for Zataria multiflora Boiss, Peganum harmala and Myrtus communis extracts were 7.4, 7.2 and 5.8 µg/ml, respectively. Although tartar emetic was more effective than plant extracts, all extracts had profound effects on promastigotes of L.major. Conclusion: Since, plant extracts of Zataria multiflora Boiss, Peganum harmala and Myrtus communis have anti-Leishmanial effects in vitro, further works are required to evaluate the exact effect of these extracts on Leishmania agents in animal models.