The objective of this experiment was to study the effect of replacing fishmeal as a protein source in tilapia diets with different percentages of soybean meal on gonadal development and fish fertility. Four diets were tested in which fishmeal was partially replaced by four different soybean meal levels (20, 30, 40 and 50%) in tilapia diets. A histological examination was performed to monitor the development of the gonads. Results showed that the final body weight was significantly affected (P< 0.05) by partial replacement of the fishmeal up to 50%. Histological examination of the gonads showed deterioration in the development of the gonads increased by increasing the dietary percentage of soybean meal. Lower replacement rates (20 to 30%) were less effective unless used for a long time, especially in males. When the replacement ratio was 40-50%, the testicular deterioration was clearly observed by the degeneration of the germinal epithelial tissue and the somatic cells that regulating the testis function. Female gonad was also affected by both low and high replacement rates, which resulted in delayed sexual maturity and increased number of degraded eggs. These negative effects can be attributed to contain soybean meal high levels of plant estrogen. As a conclusion, substituting soybean meal instead of fishmeal as a source of protein up to 50% was significantly affect growth performance of tilapia fish. Moreover, increasing the replacement ratio to 40-50%, significantly hampered the development of gonads and fertility of both sexes of tilapia due to the containment of soybean meal high levels of phytoestrogen. Therefore, it could be recommended to avoid using high levels plant protein in broadstock diets to maintain the fish fertility and fecundity.