Oxidative stress is an imbalance between oxidation and antioxidant responses caused by the accumulation of free radicals in the body. Cells produce free radicals that cause oxidative damage such as aging, atherosclerosis, arthritis, cancer, and neurological diseases. Antioxidants are substances that can neutralize free radicals, thereby reducing the damage to the body. Antioxidants are classified as synthetic and natural antioxidants. Natural products, mainly extracted from some medicinal plants, have strong antioxidant activity and low toxicity and side effects. Moreover, antioxidant activities are also detected in probiotics. Therefore, this chapter summarized the roles of antioxidants and then suggested antioxidants from natural sources have good prospects in the prevention and treatment of various diseases related to oxidative stress. In this study, antioxidant activities were mentioned in plants such as Basella alba, Cistanche sp., and representative probiotics such as Bifidobacterium breve ATCC 15700, Lactobacillus rhamnosus PN04, and Lactococcus lactis PN05 which could serve in DNA damage protection and cytotoxicity oriented for cancer treatment. With the results obtained in these medicinal herbs and probiotics, the antioxidant activities of more and more medicinal herbs and probiotics should be exploited in health care and pharmaceutical product development.