This study aimed to evaluate the effect of different protein sources, cotton seed meal (CSM), soybean (SBM) and dried distiller's grains with soluble (DDGS) in diet of growing male lambs on morpho-histological characteristics of the testis and epididymis, characteristics of epididymal spermatozoa and serum testosterone concentration. Total of 20 growing cross breed (Romanov x Rahmani) lambs (about 8 months old and 33 kg LBW) were assigned into four groups. Lambs in all groups were fed diets containing differed sources of 50% of protein in concentrate feed mixture, being sun flower meal in the 1st group (control, G1), CSM in G2, SBM in G3 and DDGS in G4 for 120 days as an experimental period. At the end of the experimental period, lambs were slaughtered and testes were collected for morpho-histological study. Blood was collected for testosterone determination in blood serum and epididymal spermatozoa were recovered by slicing. Results revealed that testicular weight and size in terms of length and width were the highest (P<0.05) in G3, moderate in G1 and G2 and the lowest in G4. Dietary protein source had insignificant effect on weight and measures of epididymis or its segments, in particular length and width of epididymal cauda testes of G3 showed higher (P<0.05) values of the largest and smallest diameters and the widest average diameter of the seminiferous tubules (ST) and the highest (P<0.05) thickness of spermatogenic layer as compared to other groups. The largest, smallest and average diameters of epididymal cauda ductules (ED) was higher (P<0.05) and the lining epithelial layer of epididymal cauda was thicker (P<0.05) in G3 and G4 than in G1 and G2. Percentages of progressive motility and livability of spermatozoa recovered from the epididymal cauda were the highest (P<0.05) in G3, moderate in G1 and G2, and the lowest in G4. Percentages of abnormality and acrosome damage of epididymal spermatozoa were not affected by dietary protein source. Serum testosterone concentration was the highest (P<0.05) in G3 and the lowest in G4. In conclusion, feeding CSM or DDGS should not be supplied to growing male lambs for breeding. Feeding diet containing sun flower meal (Control diet) or soybean meal as sources of dietary protein had save effect on growing male lambs for breeding.