Abstract: In order to explore the action mechanism of cyclophilin B in Pinctada martensii, the full length of PmCyPB gene were obtained using rapid amplification technology of cDNA ends. And expression of PmCyPB in different tissues was tested by Real-time PCR. The results showed that the total length of CyPB gene was 1266 bp, including 678 bp of the open reading frame (ORF) which encoding 225 amino acids, a 5-'UTR of 62 bp and a 3'-UTR of 526 bp. The predicted molecular weight was 24829.3, and the isoelectric point was 5.77. Multiple sequence alignment showed that PmCyPB was highly conservative in CyPB gene with other species. By SMART software analysis, we found that it contained the typical domain of the CyPB family, namely peptidylcis-transisomerase. In addition, real-time PCR indicated that PmCyPB could be expressed in all of the detected tissues, including adductor muscle, hepatopancreas, haemocytes, mantle, foot, gill, gonads, with the highest expression in mantle and the second in gill. The results could provide certain theoretical basis for the further study of PmCyPB in the immune response in mollusk.