The genes play an important role in the steroidogenesis and gonadal differentiation of teleosts. Therefore, in this study, we chose triploid and diploid rainbow trouts () at 31-68 days post fertilization (dpf) cultured in the same environment as object. In this study, triploid rainbow trouts were used as object and diploid rainbow trouts were used as controls to evaluate the expression of genes and the activity of CYP19A1B enzyme in the brain of rainbow trout. The real-time polymerase chain reaction results showed that the expression of in the diploid rainbow trout was up-regulated during 30-50 dpf and maintained a stable level during this period. Subsequently, during 50-56 dpf, the expression was down-regulated; during 56-68 dpf, the expression was up-regulated again. The expression of in the triploid rainbow trout was up-regulated during 30-35 dpf, down-regulated during 35-47 dpf, and then upregulated during 47-55 dpf; thereafter, until 68 dpf, the expression was maintained at a stable level. The expression of in diploid rainbow trout peaked at 34 dpf and in triploid rainbow trout peaked at 38 dpf. The expression of in the diploid rainbow trout was maintained at a high level during 33-42 dpf and its expression peaked at 38 dpf. The expression of in the triploid rainbow trout was high during 47-59 dpf and peaked at 49 dpf. The expression of in diploid rainbow trout peaked at 37 dpf, and then the expression was down-regulated. In the triploid rainbow trout, the peak value was recorded at 40 dpf, and then the expression was down-regulated, but the expression of in the triploid rainbow trout was lower than that in the diploid rainbow at the same period. The expression of in the diploid rainbow trout was up-regulated at 35-46 dpf and the peak value was recorded at 45 dpf, and then the expression was down-regulated until 69 dpf. However, under the same experimental conditions, the expression of in the triploid rainbow trout was not detected. The results of enzyme-linked immunosorbent assay demonstrated that at 40 dpf, the activity of CYP19A1B enzyme in the diploid and triploid rainbow trout peaked, but during 40-60 dpf, the activity of CYP19A1B enzyme in the diploid rainbow trout was significantly higher than that in the triploid rainbow trout; especially at 45 dpf and 50 dpf, the activity of this enzyme in the diploid trout was 1.15 and 1.12 times higher than that in the triploid trout, respectively. The results suggest that one of the reasons for early gonadal differentiation delay in triploid rainbow trout is the expression of is lower and later than those in the diploid rainbow trout. Moreover, estradiol cannot be synthesized normally in triploid rainbow trout, and therefore, gonadal differentiation is delayed.