Abstract: To evaluate the expression of Sox (SRY-related HMG-box) gene during development and sex differentiation of Pinctada martensii, we cloned and characterized the HMG box and the full-length cDNA of Sox gene in Pinctada martensii. DNA sequences and phylogenetic tree were analyzed. RT-PCR was used to measure the expression of Sox gene in different tissues and in gonads of different phases of Pinctada martensii. The results showed that the full-length cDNA of Sox was 1579 bp, including a 5'UTR of 126 bp, a 3'UTR of 445 bp and an open reading frame of 1008 bp, which encoded a deduced protein of 336 amino acids. Sequence comparison indicated that Sox in Pinctada martensii (pmSox11) and Sox11 in Crassostrea gigas (CgSox11) had 80% homology. Phylogenetic analysis demonstrated that pmSox11 was clustered with CgSox11. pmSox11 expressed highly in mantle, gill, foot and digestive diverticulum with more ganglia distribution and lowly in adductor muscle and female gonad with fewer ganglia distribution. Moreover, pmSox11 expressed highest in 3-month-old gonad and 1-year-old testis, modest in 1-year-old testis and 2-year-old testis, and lowest in 2-year-old ovary. The results indicated that pmSox11 might be involved in development of early nervous system and regulation of sex differentiation in Pinctada martensii.