[Objectives] To provide basis for future research on gender generation and reproductive adaptation strategies of Corbicula clams, the sex composition of C. fluminea in the section Dingcheng of Yuan River, Hunan Province, was verified, and the potential sexual transition among male, female and hermaphrodites was also explored based on their shell length. In addition, the morphological characteristics was compared between monoflagellate and biflagellate sperms. [Methods] Histological techniques were used to analyze the sex composition and the morphology of sperms. The differences of head length between monoflagellate and biflagellate sperms were compared using T-test. One-way ANOVA was used to analyze the shell length among sexes and the length differences among tails of sperms, and multiple comparisons were performed using least-significant difference (LSD). [Results] The minimum shell length at sexual maturity is 2.92 mm in males, 5.66 mm in females, and 5.30 mm in hermaphrodite individuals, respectively (n = 770), and their sex ratio was approximately 1︰1︰6 (n = 697) (Fig. 3). The average shell length of hermaphrodites (22.55 ± 0.33 mm) was significantly greater than that of the males (20.44 ± 1.03 mm) (P < 0.05) and the females (19.79 ± 0.99 mm) (P < 0.01), but there was no significant difference between males and females (P > 0.05) (Table 1). The C. fluminea population in the Yuan River can produce both monoflagellate and biflagellate sperms. The head length of monoflagellate sperms ranged from 4.93 to 21.79 μm, with a mean value of 14.27 ± 0.82 μm, and that of biflagellate sperms ranged from 10.29 to 22.04 μm, with a mean value of 15.62 ± 0.62 μm, with no significant difference between these two types of sperms (P > 0.05) (Table 2). The average length of long tails (38.07 ± 1.44 μm) was significantly greater than that of short tails (31.08 ± 1.60 μm) and the tails of monoflagellate sperms (30.15 ± 1.75 μm) (P < 0.01), while no significant difference in average flagella length was observed between the short tails and the tails of monoflagellate sperms (P > 0.05) (Table 2). [Conclusion] Our results indicated that the C. fluminea population in the section Dingcheng of Yuan River was protandrous, and with possible transition from male and/or female to hermaphrodite. There are two types of sperms:monoflagellate and biflagellate sperm with different tail length. The differences in motility and fertilization ability between coexisting monoflagellate and biflagellate sperm are worthy of further study.