Reproduction is among the most vital biological processes in the life cycle of any organism, such as fish. The biology of Oreochromis niloticus reproduction in Lake Nasser and Lake Nubia was studied using different mesh sizes of gill nets. A total of 480 Oreochromis niloticus were caught from Lake Nasser, Egypt, and 369 from Lake Nubia, Sudan, from January through December 2021. Sex ratio size at first maturity and maturity stages of O. niloticus were examined. In Nasser's Lake, the sex ratio of male to female was 1:1.02, whereas in Lake Nubia, it was 1:0.89. The first maturity length in Lake Nasser was 27.5cm for males and 27cm for females, while in Lake Nubia, it was 32cm for males and 26cm for females. The gonado somatic index indicated that the species breeds in Lake Nasser and Nubia during summer and spring, respectively.