This study was conducted to investigate the effects of brackish water (salinity=15) and fresh water (salinity=0) on gonadal development and mating behavior in adult Chinese Mitten Crab, Eriocheir sinensis. The gonadosomatic index (GSI) and hepatosomatic index (HSI) were also compared between pre-and post-mating/spawning of E.sinensis. There were four treatments in the current study, i.e. females cultured in brackish water (FB), females cultured in fresh water (FF), males cultured in brackish water (MB), and males cultured in fresh water (MF). The results showed that: (1) the survival of each treatment was around 80% and no significant difference was observed among different treatments; (2) the GSI of FB was significantly higher (P<0.05) than that of FF; although the GSI of MB was higher than that of MF, no significant difference was observed between the two treatments; (3) for the samples collected at day 30, the HSI of FF was higher than that of FB, but no significant difference was detected between two male or female treatments for other samplings; (4) at day 45, all crabs from FB and MB had mating behaviors and 66.7% of FB females spawned during two days of post-mating. Some MF crabs had estrous behaviors; (5) for the successful mating couples of FB and MB, the GSI decreased significantly after mating/spawning. In conclusion, brackish water can enhance the gonadal development and mating for adult E.sinensis, particularly for females.