The gonad of Tridentiger trigonocephalus (Gill) of the Yangtze Estuary in reproductive season was studied by lightmicroscope. The results showed that the reproductive season of T. trigonocephalus was April, May and June in every year. The ovaries in reproductive season were mostly in stage Ⅳ. From April to June, the ovaries in stage Ⅳ account for 62.32%, 71.43% and 48.41% of the total amount in every month respectively. The ovaries in stage Ⅲ account for 24.55%, 12.33% and 6.28% of the total amount in every month respectively. The ovaries in stage Ⅴ account for 13.13%, 10.45% and 26.67% of the total amount in every month respectively. The ovaries in stage Ⅵ account for 5.89% and 18.10% in May and June respectively. The corresponding gonadosomatic index (GSI) of ovaries from stage Ⅲ to Ⅵ were 4.49%±2.07%, 31.75%±7.08%, 38.33%±6.24% and 7.78%±3.18%. The testes were in stage Ⅳ, Ⅴand Ⅵ from April to June. The testes were mostly in stage Ⅳ in April， in stage Ⅴ in May and June. The corresponding GSI of testes in stage Ⅳ, Ⅴ and Ⅵ were 0.66%±0.07%, 0.96%±0.14% and 0.43%±0.06%. The oocytes could be divided into five phases. In the ovaries at stage Ⅲ, the oocytes were mostly at phase Ⅲ, follicular membrane and yolk grain appeared in the cytoplast. There were few oocytes at phase Ⅰ and Ⅱ at the same time. In the ovaries at stage Ⅳ, the oocytes were mostly at phase Ⅳ. The oil ball began to appear in the oocytes at the late phase Ⅳ. nucleus were excursion and distortional，zona radiate was clear at the same phase. In the ovaries at stageⅤ, the oocytes were dissociating. The two layers of follicular membrane which had outsided the plasma membrane were falling off. The ovaries at stage Ⅵ were mostly made of the oocytes at phase Ⅱ, Ⅲ and lots of discharged follicles. The testes of T. trigonocephalus were lobular type. All stages of spermatogenic cells were in the spermatogenic cysts. The wall of the spermatogenic cysts was made of sertoli cells. Spermatogenic cells in the same spermatogenic cysts developed synchronously. As spermatogenic cysts ruptured and released the sperm into lumen of seminiferous lobula after spermatozoa have matured, the developing process of spermatozoa would be completed.