Aesculus hippocastanum L. is a native tree of Asia. Its leaves, seeds and flowers have long been used in folk medicine and in traditional food ingredients. In this study, the bark, seeds, leaves and flowers as aerial parts of A. hippocastanum were extracted in ethanol. The antioxidant capacity of each part was determined for its 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging capacity, microsomal lipid peroxidation inhibition capacity and total phenolic content. Among all the parts examined, the bark extract of A. hippocastanum revealed the highest antioxidant capacity with an IC50 value of 0.025 mg/mL and 0.014 mg/mL for the inhibition of lipid peroxidation and for the scavenging of DPPH radical, respectively. The bark extract was further examined for its cytotoxic effect on human breast cancer cells (MCF-7) and on healthy cells (3T3) using the MTT method. Cell viability was reduced to 30% upon the addition of 0.5 mg/mL bark extract for both cell lines.