Monosex, all female or all male progenies, is important in fish production to increase the benefits to the aquaculture industry. Cobia (Rachycentron canadum) is a potential candidate for sea-cage culture in Taiwan. In this study, the sex steroids estradiol (E2) and methyltestosterone (MT) were added at various doses to the feed of cobia to control its sex. Gonadal sex differentiation of cobia larvae was apparent 50-70 days post-hatching (dph). Gonadal differentiation was promoted by the administration of sex steroids. The feminization rate of cobia fed a commercial feed with E2 added at doses of 5 and 50 mg kg-1 was 95% and 100%, respectively; the feeding periods were 55-120 dph and 55-99 dph for the low- and high-dose treatments, respectively. The masculinization rate of cobia given feed with MT added at doses of 25 and 75 mg kg-1 was 100% in both cases for a feeding period of 45-100 dph. In conclusion, our results show that the sex of cobia can be controlled by exogenous sex steroids and our protocol can be used in the breeding and reproduction process of cultured cobia