Selenium (Se) is an essential trace element which shows fundamental and important roles in medical health and industrial manufacturing. Selenium is normally a redox sensitive element that occurs in different forms including selenide [Se(?2)], elemental selenium [Se(0)], selenite [Se(+4)], and selenate [Se(+6)], all of which present in nature and organisms as organic or inorganic forms. Microorganisms play an important role in selenium transporter, reduction, oxidation, assimilation, and methylation in the environment. This review summarizes the importance of microorganism on the biogeochemical cycle of selenium, include: 1. Selenium cycle in environment; 2. Mechanism of selenium metabolism in microorganisms.