Abstract: In order to obtain all male population, the Nile tilapia (Oreochromis niloticus) larvae which just finished the yolk sac were fed with feed containing 17α-Methyltestosterone at five different doses (0, 20, 30, 40, 50 and 60 mgMT kg-1 feed) for 28 days. As a result of the experiment repeated twice, it was found out that the sex reversal rates among the groups were at the levels of 57.1, 69.8, 69.4, 70.9, 86.1 and 93.7%, respectively. Furthermore, it was observed that the highest sex reversal rate occurred in the group to which 60 mgMT kg-1 feed dose was applied.