Selenoprotein P (SeP : the "P" denotes its presence in plasma) is the major selenoprotein in plasma. All selenoproteins so far reported contain one atom of selenium (Se) as selenocysteine per molecule except SeP, which is thought to contain ten atoms of Se as selenocysteine per molecule. SeP contains 50% of the total Se in plasma. The function of Sep is currently unknown, although several indirect lines of evidence suggest that SeP is a free radical scavenger or Se transporter. Recently, we reported that SeP is capable of reducing phospholipid hydroperoxide in the presence of glutathione, and characterized the enzymatic nature of SeP. SeP is also reported to function as a peroxynitrite scavenger or cell survival factor in primary culture of neurons. In this minireview, we discuss the relationship between the structure and function of SeP.