Leishmania parasites, which are reported to be endemic in 98 countries around the world, infect humans as well as wild and domestic carnivores and small mammals, and are transmitted by sand flies (Phlebotomus, dwarf sandflies). It is reported that 350 million people are at risk and two million new cases are seen in the world every year. It has been reported that different drugs (topical paromomycin, oral miltefosine, ketoconazole, rifampin, and zinc) have been tried in studies especially in endemic regions in the treatment of cutaneous leishmaniasis, and response to treatment has been obtained at different rates. Today, the search for alternative treatments continues and many studies have been carried out for this purpose. For centuries, olive leaf extracts have been used to maintain health. Oleuropein has numerous health benefits, including antioxidant, antimicrobial, anti-inflammatory, antiatherogenic, anticarcinogenic, antiviral activities, cardio- and neuroprotective, hepatoprotective effects. The aim of this study was to determine and understand the mode of action of oleuropein, the cell death mechanisms caused by oleuropein in L.tropica promastigotes. In this study, the phenolic and flavonoid content of oleuropein was determined by HPLC method. The antioxidant capacity and the amount of oleuropein were determined. Afterwards, morphological and physiological (mitochondrial membrane potential, formation of reactive oxygen species, Annexin V binding) changes triggered by oleuropein in L.tropica promastigotes were investigated using flow cytometry. Our studies revealed that apoptotic properties such as mitochondrial dysfunction, production of reactive oxygen species, flip-flop action of phosphatidylserine could induce cell death in L.tropica promastigotes. It has been observed that oleuropein induced typical apoptotic morphological features in L.tropica promastigotes. Total phenolic content and total flavonoid content values of oleuropein extract were determined as 33 mg/g and 229 mg/g. The radical removal method was used to investigate the antioxidant capacity of methanol extracts against free radicals. Total antioxidant content of oleuropein extract was determined as 87%. In addition, the amount of oleuropein in the oleuropein extract was determined as 21. 1% by HPLC. The oleuropein dose that killed 50% of L.tropica promastigotes, that is the IC50 value, was detected as 46.6 µg/mL after 24 hours. It was observed that the parasites in the control group preserved their typical morphological features with a single nucleus, flagella, kinetoplast and narrow cell body at both 24 and 48 hours. It was observed that as oleuropein concentrations increased, the and kinetoplasts of L.tropica promastigotes could not be distinguished from each other, they moved away from the narrow cell body structure, they lost their flagella and turned into a round form, and they moved away from the typical form of the parasite. The percentage of Annexin V+ apoptotic cells was found to be 2.9 ± 0.4% in the untreated control group, and 38.1 ± 6.9% in the oleuropein-treated group. Polarization in the mitochondrial membrane of healthy promastigotes caused an approximately 1.7-fold change in the direction of depolarization in oleuropein-treated promastigotes. According to these findings, oleuropein triggered mitochondria-related death in L.tropica promastigotes. Moreover, 1.4 ± 0.2 fold increase in reactive oxygen species production was detected in oleuropein-treated promastigotes compared to the untreated control group. Comparisons between groups were made using the independent sample t test method. In conclusion, phenolic compounds of olive leaf extract oleuropein induced apoptotic cell death in L.tropica promastigotes. Our results support that olive products such as oleuropein may have anti-parasitic effects.