Antioxidants are substances at low concentrations and with a special mechanism which slow down or stop oxidation processes. Antioxidants are divided into two synthetic and natural categories. In this study, plants are the most important sources of antioxidants that could protect cells from oxidative damage which can lead to cell death and tissue damage. Antioxidant enzymes increase the potency of antioxidants and reduce the incidence of certain diseases, such as cancer, cardiovascular disease and stroke. There are two general groups for determining the antioxidant activity. These methods including the method for the hydrogen atom transfer reaction mechanism and method for the hydrogen-electron-single-electron transport reaction mechanism.