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Background: There is growing evidence that damage to spermatozoa by reactive oxygen species (ROS) plays a key role in male infertility. This study was done to review the role of oxidative stress in male infertility. Materials and Methods: In this review article, PubMed, Scopus, and EBSCO-CINAHL databases were used for finding the relevant studies. Results: Under physiological conditions, a certain level of ROS is necessary for normal sperm function. However, an excessive level of ROS produced by leucocytes and immature sperms can cause damages to spermatozoa. Oxidative stress develops when there is an imbalance between ROS production and antioxidant defense system in male reproductive tract. High levels of ROS have been detected in the semen samples of 25-40% of infertile men. Oxidative stress can induce detrimental effects on standard seminal parameters and fertilizing capacity of spermatozoa. Conclusion: Oxidative stress can induce impaired sperm function that results in poor pregnancy rate in natural conditions and assisted reproduction.