To verify the sex steroids which are involved in oocyte maturation of the blacktip grouper, $Epinephelus$ $fasciatus$, we incubated vitellogenic oocytes (0.41 and 0.50 mm in average diameter) in the presence of exogenous steroid precursor ($[^3H]17{alpha}$-hydroxyprogesterone). Steroids were extracted, separated and identified by thin layer chromatography. The major metabolites produced were androstenedione, estradiol-$17{eta}$, estrone and progestogens. Progestogen metabolites in the oocytes of 0.50 mm were more abundant than those of 0.41 mm. Also, we investigated the $in$ $vitro$ effects of human chorionic gonadotropin (HCG; 5, 50 and 500 $IU/m{ell}$), $17{alpha},20{eta}$-dihydroxy-4-pregnen-3-one ($17{alpha}20{eta}P$) and $17{alpha},20{eta}$-trihydroxy-4-pregnen-3-one ($17{alpha}20{eta}21P$; 5, 50 and 500 $ng/m{ell}$, respectively) on oocyte maturation. In the oocytes of 0.41 mm, treatment with 50 IU HCG stimulated GVBD ($55.30{pm}1.20%$) compared with controls ($32.41{pm}3.13%$, $p$<0.05). In the oocytes of 0.50 mm, treatment of $17{alpha}20{eta}P$ (50 and 500 $ng/m{ell}$) stimulated GVBD ($50.13{pm}2.52$ and $51.77{pm}5.91%$, respectively) compared with controls ($36.81{pm}2.89%$, $p$<0.05). Treatment with 500 IU HCG also stimulated GVBD ($49.59{pm}5.15%$) compared with controls ($p$<0.05). Taken together, these results suggested that both HCG and $17{alpha}20{eta}P$ were effective on in vitro oocyte maturation and $17{alpha}20{eta}P$ may act as a maturation inducing hormone in blacktip grouper.