Cytochrome P450 aromatase (P450arom) catalyzes the conversion of androgens to estrogens and plays an important role in reproduction and development in vertebrates. We investigated the expression patterns of ovarian P450arom (P450aromA) and brain P450arom (P450aromB) mRNA during sex change in black porgy. Maturity was divided into seven stages from male to female (immature testis, mature testis, testicular portion of mostly testis, ovarian portion of mostly testis, testicular portion of mostly ovary, ovarian portion of mostly ovary, and mature ovary). P450aromA expression was significantly higher in the ovarian portion of mostly-ovarian stage fish, and P450aromB expression was highest in the brain of black porgy with mostly-ovarian gonads. Histology showed that testicular tissues were disintegrated with the development of ovarian tissue associated with an increase in the expression of the two P450arom mRNAs during sex change. Interestingly, among various tissues, P450aromA was only expressed in the ovary, and P450aromB was only expressed in the brain. To understand the role of gonadotropin-releasing hormone (GnRH) and estradiol ($E_2$), we injected exogenous hormone (GnRH analogue [GnRHa] and $E_2$) into immature black porgy. In the GnRHa group, expression of the two P450arom genes decreased 12 h after injection, and expression of the two P450arom genes were significantly higher at 6 dafter $E_2$ injection. These results provide useful baseline knowledge on the mechanism of natural sex change in black porgy.