Antileishmanial drugs used in the treatment of leishmaniasis are toxic and expensive. Moreover, parasites have recently developed resistance against them. Hence there is an increasing need for developing new antileishmanial medicines. Quercetin, found in the roots, leaves and fruits of many plants, is a natural polyphenolic flavonoid. Quercetin has antibacterial, antiviral, anti-carcinogenic, and antioxidant properties. On the other hand, because of its weak solubility in water, quercetin has had limited use on humans. To increase its bio-availability and maximize its therapeutic effects, quercetin has recently been encapsulated with nanoparticulate carrier systems. The aim of this study is to encapsulate quercetin in biodegradable, bio-compatible poly-ε-caprolactone (PCL) nanoparticles, to characterize the synthesized nanoparticles and to analyze their in vitro antileishmanial efficacy on L.infantum parasites. Quercetinloaded PCL nanoparticles (QPNPs) were synthesized using oil-in-water single emulsion solvent evaporation method. Their characterization was done using scanning electron microscopy (SEM) and dynamic light scattering (DLS) equipments. Encapsulation effectiveness and release profiles of QPNPs are calculated with UV-Vis spectrophotometry. The antileishmanial effectiveness of the synthesized nanoparticles was analyzed in L.infantum promastigote culture and amastigote-macrophage culture. The results indicated that QPNPs had an average size of 380 nm, a zeta potential of -6.56 mV, and a PDI value of 0.21. The measurements showed the quercetin-loaded nanoparticles to have an encapsulation effectiveness of 64% and a reaction efficiency of 55%. After an incubation of 192 hours, nanoparticles were seen to release 58% of their quercetin content. The synthesized QPNPs had IC50 values on L.infantum promastigotes and amastigotes of 86 and 144 µg/mL respectively. This means that QPNPs have reduced the vitality of promastigotes about 20 times and of amastigotes about 5 times as compared to the control group. These results demonstrate the strong antileishmanial potentials of QPNPs. It is believed that if these positive findings are supported by further in vivo studies, QPNPs may be used in the treatment of leishmaniasis.