The present study determined C-values of 23 individuals of Yangtze finless porpoise Neophocaena phocaenoides asiaeorientalis by flow cytometry using chicken erythrocyte DNA content as a standard. The Yangtze finless porpoise's lymphocyte cells were extracted from whole blood samples stored at three different conditions. For determining the actual C-value of chicken erythrocyte, we firstly standardized its C-value controlled by human C-value of 3.50 pg/C, and then the chicken C-value of 2.35 pg/2C was used in the following determinations. Result shows that the C-value of the Yangtze finless porpoise is 3.27 pg with the genome size of 3.17 x 10(9) bp. The C-values for male and female are 3.29 pg and 3.25 pg, respectively. The C-values of Yangtze finless porpoises reared in captivity and wild are 3.06 pg and 3.30 pg, respectively. Analyzed by Independent-Samples T Test, it's found that the C-values of different genders have no significant difference, whereas, the C-values of wild Yangtze finless porpoises and that reared in captivity have significant differences, the C-value of Yangtze finless porpoise in captivity is smaller than that of the wild ones. It's assumed that the stability of environmental factors has certain influences on animal genome size variation.