Selenium(Se) is a naturally occurring trace element. It is essential for humans and animals but is very toxic at higher concentrations and will make the selenium pollution. In four inorganic states of selenate［SeO₄^2- (VI)］, selenite［SeO₃^2- (IV)］, elemental selenium［Se° (0)］ and selenide［Se^2- (-II)］, selenium oxyanions including selenite and selenate are well known to be more bioavailable but higher toxic than other forms. Many bacteria have the capacity to reduce selenium oxyanions to red elemental ground granule, called red elemental selenium, which is likely to be Se-protein compound. It has lower toxicity comparing with selenite, but when the size is at 5～200 nm, its bioavailability to selenite was similar in terms of inducing seleno-enzymes in cultured cells and in Se-deficient rats. This provides the potential to gain the best manner to supply selenium for organisms and solve the pollution of selenium. So far, series of researches have been made to study this bacterial reduced process. Many substances such as carbon source, oxygen, elemental sulfur, glutathione, some oxidoreductases and transfer protein in membrane were found to affect or participate this biological dissimilatory reduction. This review presents the discovery of the biomorphic red elemental selenium, factors affecting the bacterial reduction of the selenium oxyanions, and the possible pathways to form red elemental selenium in bacteria.