Cobia (Rachycentron canadum) is one of potential candidates for sea-cage culture species in Taiwan. At present, due to the inbreeding depression of cobia, the quality of seeds is decreasing, including biological characteristics, deformity, growth rate, etc. We want to solve these problems by using selected breeding method to improve the quality of cobia seeds. In this study, the cobia fingerlings were fed with E2 diets(5 mg kg–1, 50 mg kg–1 ) from 55DPH to 120DPH, feminized the undifferential cobia to 100% female and confirmed timing of gonadal sex differentiation. Results showed that cobia gonadal sexual differentiation was during 50-70 DHP and 50mg E2 kg–1 diet induced cobia fingerlings to 100% females. E2 also caused cobia to grow slowly and lower feed conversation efficiency(FCE). After E2 treatment with 66 days, cobia were feeding normal diets and grew better than before. Follow-up cultivated for seven months, the weight of female cobia was obviously superior to male. Finally, steroid hormone can really control sex of cobia and was applied to reproduction of all female progeny. These results can be applied to cobia aquaculture.