Histological study was conducted to determine the initial treatment time and treatment period of hormone for sex reversal in accordance with gonadal development and sexual differentiation in Oreochromis niloticus. The effects of various concentrations and various treatment periods of 17$alpha$-methyltestosterone (MT) on sex reversal, growth, and condition factor were also evaluated. Paired primodial gonads were formed 9 days after hatching, when germ cells began their gradual multiplication and development into gonial ones. Sex differentiation of gonads either into ovaries or testes became histologically discernible about 20 days after hatching with formation of ovarian cavity and efferent duct. All feed treated with MT at 15 ppm for 10 days or more produced populations of males $95\%$ or above. All male populations were produced at 15 ppm MT for 40 days, and 30 ppm for 30 and 40 days. Growth of hormone-treated-fish was faster than that of untreated ones and the condition factor of hormone-treated-fish was greater than that of untreated ones 77 days after hatching.