In this study, oocyte steroidogenesis are investigated in relation to oocyte development in the starry flounder, Platichthys stellatus, a marine multiple spawner. Vitellogenic (0.52 and 0.55 mm oocyte diameter) and mature oocytes (0.63, 0.66 and 0.71 mm oocyte diameter) were incubated in vitro in the presence of $[^3H]17{alpha}$-hydroxyprogesterone ($[^3H]17{alpha}$-OHP) as a precursor. Steroid metabolites were extracted from the incubated media and oocytes, the extracts were separated and identified by thin-layer chromatography (TLC), high performance liquid chromatography (HPLC) and gas chromatographymass spectrometry (GC-MS). The major metabolites produced from $[^3H]17{alpha}$-OHP were androgens [androstenedione ($A_4$) and testosterone (T)] and estrogens [$17{eta}$-estradiol ($E_2$) and estrone ($E_1$)] and progestins [$17{alpha},20{alpha}$-dihydroxy-4-pregnen-3-one ($17{alpha}20{alpha}P$) and $17{alpha},20{eta}$-dihydroxy-4-pregnen-3-one ($17{alpha}20{eta}P$)] in vitellogenic and mature oocytes. The results from this study suggest the potential roles of $E_1$ in the oocytes with diameter 0.52-0.71 mm, $17{alpha}20{alpha}P$ and $17{alpha}20{eta}P$ at the oocytes of 0.63, 0.66 and 0.71 mm.