Recently, the new epigenetic marker, 5-hydroxymethylcytosine (5hmC) has been widely studied. 5hmC derived from the enzymatic oxidation of 5-methylcytosine (5mC) by TET(ten-eleven translocation) protein family and called as the "six base" of higher organism genomic DNA. It is found that 5hmC not only influence the structure and function of genome, but also play an important role during early embryonic development. This review mainly discussed the research advances of 5hmC in metabolic pathways, biological function, genome distribution and analysis methods.