Methyl farnesoate ( MF) biosynthesized and secreted by mandibular organ( MO) in Chinese mitten crab ( Eriocheir sinensis) , including precocious and normal crabs, was investigated by using in vitro radiochemical method. It was found that MF biosynthesis rates of precocious crabs ( female and male) in September were about two times higher normal juvenile crabs, and reached the peak levels in October or November( 1st autumn) . After that, it decreased signif icantly , but GSI continued to increase during the winter. In normal crabs, MF biosynthetic rates maintained much lower levels from September( 1st year) to April ( 2nd year) w ith undeveloped gonads. Since June ( 2nd year) , MF biosynthetic rate increased rapidly. In October ( 2nd year) , MF biosynthet ic rates in both female and male were beyond the peak levels of precocious crab. In November, MF decreased signif icant ly, while gonads reached the end of stage .. The results indicated that MO development in advance in the precocious crabs( female and male) , made large amount of MF to be biosynthesized and secreted and then stimulated ovaries and testes development and maturation. That is the endocrine factor for causing precociousness of crab.