Individuals of Portunus trituberculatus were collected from Shipu Port, Xiangshan, Zhejiang Province from March 2033 to June 2004. The chromosomal number and karyotype of the swimming crab were studied by the tissue slice techniques applied to the mature oocytes and the air-drying method applied to the testes, embryos and zoeae. The spread plates were examined and the chromosomes at the mitotic or meiotic metaphase were photographed under an Olympus microscope. The chromosomes pairing were processed according to the relative lengths and the centromeric indexes. The chromosome pairs was classified following the method of Levan et al （ 1964）. The results indicate that the testes are the best materials to be used in chromosome count and the egg-zoeae are the best materials to be used in karyotype analysis. The chromosomal numbers for P. trituberculatus are 2n = 106, n = 53. All chromosomes （2n= 06） were matched in 53 pairs. They were divided into 3 groups. Group Ⅰ contains 20 pairs （ lst- 20th） that are metacentric chromosomes（m）. Group Ⅱ has only 3 pairs （21st - 23rd） of submetacentric chromosomes（ sin）. And group Ⅲ includes 30 pairs （24th - 53rd） which are telocentric chromosomes（t）. Therefore, the karyotype formula of P. trituberculatus is 2n = 106 = 40m ＋ 6sm ＋ 60t, NF= 152. No sex-chromosome of heteromorphism was observed.