The extracts of thoracic ganglion from adult Macrobrachium rosenbergii were chromatographed on a Sephadex G-50 column, the resulting second peak was further isolated by HPLC, and 9 fractions were obtained. Among the fractions only the first fraction significantly increased the oocyte diameter in vitro of the stage Ⅲ ovary in M. rosenbergii. The increasing effects were depended on the protein concentration of the fraction for culture. The molecular weight of the substance in the fraction 1 was about 1.3kDa.The results show that the thoracic ganglion of M. rosenbergii can secrete peptide hormone which stimulates the oocyte development of the prawn.