In humans, oxidative stress and antioxidant defenses are the sum of a complicated network of enzymatic and non‐enzymatic processes. Depending on the stage and severity of diseases, a patient’s antioxidant armamentarium may increase as an appropriate response to an oxidant challenge, whereas others may decrease as an indication of unbalanced consumption. In some cases, the formation of reactive oxygen species is a requisite and healthy event. In fact, free radicals can affect intracellular signal transduction and gene regulation, resulting in cytokine production essential to the inflammatory process. In many other cases, especially liver diseases, excessive oxidative stress undoubtedly contributes to the progression and pathological findings of disease and serves as a prognostic indicator. Reactive oxygen species are highly reactive molecules that are naturally generated in small amounts through metabolism and could damage cellular molecules such as lipids, proteins or DNA. Oxidative stress plays a major role in many liver diseases. In this review, we summarize the biological character of free radicals and some antioxidants, and the related methods of analysis. Then, we discusses the association of oxidative stress to many types of liver diseases.