Nanosized materials are of increasing interest due to their optical and electrical properties. In particular, the semiconductor ZnO has a wide band gap of 3.3 eV, which is abundant in nature and is eco-friendly. Green synthesis of nanomaterials is simple, non-toxic and adapted to large-scale production. Here we synthesized ZnO nanoparticles using the fruit shell extract of horse chestnut, Aesculus hippocastanum. ZnO nanoparticles were characterized by X-ray diffraction (XRD) and field emission-scanning electron microscopy. XRD patterns were indexed on the basis of an hexagonal structure, and the pattern indicates high crystalline quality with very well-defined high intensity peaks. The grain sizes ranged between 50 and 100 nm. Ultraviolet-visible data show that ZnO nanoparticles have a high optical transparency of 70-86%. In addition, the bactericidal effect of ZnO nanoparticles was observed against Bacillus thuringiensis.