Morphological and histological studies of imposex has been studied in the oyster drills, Thais clavigera and T. rufotincta. The sex ratios of the collected samples were close to 1:1 among sites of Shunsun, Taishi, Budai, Chiku and Donkang. The relative penis sizes (RPS values) of T. clavigera were higher than T. rufotincta, indicating a high imposex degree in T. clavigera. The RPS values of T. clavigera showed seasonal variations, because the penis length of males and females were longer in the reproductive season, but the trends were inconsistent among the sampling sites. The RPS values in Shunsun and Taishi were higher in March (reproductive season) and lower in August (non-reproductive season), but Budai and Chiku were in a contrary way. It is suggested that the inconsistent patterns may result from temporal and spacial variations of tributyltin (TBT) and triphenyltin (TPT) concentrations among sites. For imposex females, they all developed penis and vas deferens. In the heaviest affected female T. clavigera, aborted egg capsules within the capsule gland that renders the female sterile has been observed. The percentage of capsule gland blockage in Shunsun, Taishi, Budai and Chiku were 7.7%, 9.7%, 50% and 0%, respectively. Other accessory sex organs, i.e. the lengths of capsule gland and prostate glands were all longer in reproductive season, and the difference among sampling sites were also significant. In histological studies of T. clavigera and T. rufotincta, the male testis tissue from all sampling sites contained all developing stages sperm cells in March; as females from March, the ovary tissues were full with mature oocytes. However, in Shunsun and Budai, there were many hemocytes found in the ovary tissues of T. clavigera that might result from imposex. In lab-experiments, T. clavigera were treated with TBT and TPT for 44 days, there was no difference in female penis length between control and TBT or TPT treated groups. The ovary and testis tissues were all developed normally. In general, there is no obvious abnormality in imposexd T. rufotincta. On the contrary, in T. clavigera, the female opening (vulva) occluded by overgrowing vas deferens which might prevent the spawning of egg capsules. In addition, some hemocytes were found in the ovary tissues in T. clavigera and the phenomenon of sex change was not observed. Meanwhile, no obvious abnormality was observed in the testis tissues of T. clavigera.