The morphology and behavior of chromosomes in Eriocheir sinensis and Eriocheir japonica hepuersis spermatocytes during meiosis, as well as SC. karyotype of them at pachytene were studied cmparatively, by light or electron microscope, using the methods of air-drying and air-liquid surface spreading. The results show that: leptotene: chromosomes appeared as threads and each of them was oriented, with one or two ends, in the region of nuclear membrance. Zygotene: homologous began to pair and synapse in the position of telomeres and kinetochores, then segments of SC. formed. Pachytene: paring between homolgues was over, 73 SCs (synaptonemal complexes), were observed in most of primary spermatocytes of the both species, no any allosynapsis or heterosynapsis was observed. SC. karyotype of E. sinensis was n = 73 = 11m + 7sm + 142 + 10t + 31T and that of E. japonica hepuersis was n = 73 = 8m + 11sm + 14st + 14t + 26T. Two NORs were observed in each of the both species, on the homoloques NO. 38、39 in E. sinensis and on the No.3、12 in E. japonica hepuersis. Diplotene: homologues began to disconnect, and terminalization of chiasmata began too. Because of difference of number and position of chiasmata in each of the bivalents, the bivalents presented various shape. One heterotypical bivalent was observed in E.japonica hepuersis. Diakinesis: homologues disconnected continuously, the sites of chiasmata in most of the bivalents had moved to the ends. Metaphase I: Chromosomes condensed in greatest degree. most of the bivalents were dots or bars or dumbbells in shape, chiasmata were observed obviously in a few of the bivalents in both species only, these bivalents were "-O-" in shape. Metaphase II: sister chromotids began to disconnect. chromosomes weredots or bars or dumbbells in shape. The lengths of ten complete sets of SCs were measured and the t-test of two neighbouring SCs were done for each of the two species. the t-test of SCs with the same sequence number in the two species were dome too. The results showed that there was no significant deviation between neighbouring SCs, 21 pairs in E. sinensis and 35 pairs in E. japonica hepuersis, and between SCs with same sequence number in the two species in size. Finally, the sex-determining mechanism of the two mitten crabs, the characteristics of morphology and behavior of meiosis chrosomes, and cytogenetical difference between them, etc. are also discussed in the paper.