The present study investigated the gonads alterations of mature female Oreochromis niloticus caused by different concentrations (0, 40, 60, 100 μg/L) of 4-nonylphenol (NP) and also its effect on steroidogenesis. A tendency for a dose-dependent reduction in the gonadosomatic index, fecundity and oocytes diameter in the NP-exposed groups was observed. Histological examination revealed that NP impairs gonadal growth clearly shown in the oocyte development and differentiation. Also, the estrogenic activity of such NP was identified through 11-ketotestosterone, 17b-estradiol and vitellogenin which were affected significantly by the applied concentrations. The study indicates that NP has estrogenic potency-induced marked alteration in the sexual development including gonadal maturation, spawning time and egg production.