The present study aimed at investigates the hematological characteristics of monosex male Nile tilapia, Oreochromis niloticus, induced by feeding on 17 α methyltestosterone (17-α-MT). A total number of 2000 fry were divided into two groups based on the exposure time (15 and 30 days). Each group was subdivided into three groups based on the concentration of 17-α-MT in the feed. First (control) group fed basal diet 0 mg 17-α-MT /kg food, group 2 fed 40 mg 17-α-MT /kg food and group 3 fed 60 mg 17-α-MT /kg food. Fish in all groups were cultured under similar feeding regime and stocking density for six months. At 5th, 5.5 th and 6th month of age, the sex ratio, total red and white blood cells count, hemoglobin (Hb) concentration, HCT and the total and differential leukocytic count were determined. The erythrocytic indices were also calculated. The obtained results showed that there was a significant (p < 0.05) increase in male ratio in treated groups. There was a significant decrease in total RBCs count Hb concentration of treated males compared with the control males at the different experimental ages. However, treatment with 17-α- MT caused a significant increase in MCV and MCH in 40 and 60 mg treated males when compared with the control males at the different experimental ages. HCT and MCHC not significantly changed. While, treatment with 17-α-MT caused no significant variation in the total and differential WBCs count. It could be concluded that induction of all male monosex Nile tilapia by using 17-α-MT caused changes.