The antioxidant and antigenotoxic effects of acetonic, ethanolic, and methanolic extracts of onion peel in non-cellular and cellular systems were evaluated. HPLC analysis was performed to determine the amounts of flavonoids in onion peel extracts. HPLC analysis showed that onion peel extracts contained well-known antioxidant compounds, including epicatechin and morin. All of the flavonoids measured and oxygen radical absorbance capacity (ORAC) values were greatest in the methanolic extract. ORAC was strongly correlated with total phenolic content ($r^2=0.980$). The total radical trapping antioxidant potential was greatest in the ethanolic extract. DPPH radical-scavenging activity was in the order of acetonic>ethanolic>methanolic extract. In contrast, the reducing capacity of intracellular oxidative stress was almost the same in all extracts. In addition, pretreatment with onion peel significantly reduced human leukocyte DNA damage induced by $H_2O_2$ or 4-hydroxynonenal. Taken together, the results indicate that onion peel has antioxidative and antigenotoxic properties.