Abstract: The chromosome karyotype, DNA content of nucleus of erythrocyte and the characteristics of gonad development were studied to explore the ploidy and fertility of the hybrid F₁of Ctenopharyngodon idellus♀×Squaliobarbus curriculus♂ in reproduction and non-breeding seasons. The results shown that the number of chromosome was 2 n=48, and that the karyotype formula was 2 n=26 m+20 sm+2 st, NF=94, and that the terminal chromosome, satellite and the secondary constriction were not observed. The DNA content of erythrocyte in hybrid F₁ was (3.21±0.61) pg/N that was 1.29 times compared with chicken. The results of anatomical observation and histological section showed two types of ovary. One type was normally developed with different developmental stages of oocytes that could be used to produce next generation; the other type could develop to phase Ⅳ and gradually became abnormal in phase Ⅴ with the degraded yolk granules or without egg yolk. The testis-type of gonad mainly included primary spermatocytes, secondary spermatocytes and spermatozoons, but without mature sperm; partial lobules were full of connective tissue or adipocyte and few androcytes. Until now, semen had not been found in the production practice. In conclusion, the hybrid F₁ is diploid; partial female is fertility and the male may be infertility.