Abstract: Sex hormones are involved in animal sex, gonad development and reproductive cycle regulation and other functions, it"s known that 17beta-hydroxysteroid dehydrogenase (17β-HSD4) had played an important role in synthesis and metabolism of the sex hormone. 17β-HSD4 mainly catalyzes to deactivate the biological activity of estradiol and androstenediol. In the present study, we got a partial expression sequence tag of 17β-HSD4 from the transcription group database of Chlamys farreri firstly, then a cDNA was cloned by using Rapid Amplification of cDNA End (RACE) methods, of which the full-length is 2217 bp, contains an open reading frame of 2223 bp and encodes a peptide of 740 amino acids. Homology analysis and phylogenetic tree revealed that it"s 17β-HSD4. The mRNA was detected widely in the testis, ovary, adductor muscle, mantle, gill, hepatopancreas and kidney, especially higher in hepatopancreas and kidney. The result of qRT-PCR showed the expression of 17β-HSD4 gene in testis at mature stage is obviously higher than the testis at proliferative stage and resting stage, and ovary of all stages. It could be concluded that 17β-HSD4 can be synthetized in many tissues of Chlamys farreri, also might regulate the maturity of testis.