The effect of water temperatures in combination with or without hormone 17α methyltestosterone application for different periods (1, 2, 3 and 4 weeks) on sex ratio and survival rate of O. niloticus fry were investigated. Three rearing temperatures (25, 30 or 30°C) were tested; within each rearing treatment two levels of 17α methyltestosterone (0 or 60 mg/kg basal diet) were used. Sex ratio and survival rate of O. niloticus fry were determined after 1, 2, 3 and 4 weeks. Therefore, 24 groups (3 × 2 × 4) were tested.  At the end of this experiment, sex reversed fry were chosen at random from each group and stocked in 72 L aquaria (3 replicates for each group, 3×24) and used to investigate the effect of rearing water temperature in combination with or without hormone application for different periods (1, 2, 3 and 4 weeks) on growth performance and feed utilization of O. niloticus fry during one month after treatment period. The highest final body weight (BW), body length (BL), weight gain (WG), specific growth rate (SGR) and the worst feed conversion ratio (FCR) were obtained for fry reared (pre-exposured) at 35°C and fed on the diet supplemented by 60 mg 17α-MT for one week and the lowest final BW, BL, WG, SGR and the best FCR were recorded for the control fry group (reared at 25°C and fed on the diet free from 17α-MT for two weeks) and the same trend was also observed for tilapia fry pre-exposured to each of water temperature and 17α-MT for three or four weeks. For the control, fry group rearing periods (1, 2, 3 or 4 weeks) did not affect the final BW, BL, WG, SGR or FCR of growing the Nile tilapia fry while, increasing the rearing period for C35H1 gradually increased these parameters.