The main objectives of the present experiment were to investigate the effect of biological treatment (fungus + yeast) on chemical composition and utilizability of treated (corn cobs) in sheep ration. Twenty growing cross bred (Rahmani × Ossimi) male lambs of  4 months old and mean body weight 28 kg were distributed into four similar groups. The treated corn cobs were to replace 0, 10, 20 or 30% of Berseem hay included in the feed mixture fed to growing lamb groups R₁, R₂, R₃ and R₄, respectively. The experimental period lasted for 90 days. Feed mixtures were offered daily at 3% of live body weight and groundnut vines were ad-lib.Results indicated that, the average body weight gain, feed intake and feed efficiency differed significantly higher for sheep fed R₄ followed by R₃. However, no significant differences were detected between R₂ and the control ration. Feed efficiency values were significantly between groups and the highest values were recorded with R₄ and R₃. Digestibilities of DM, OM, CP and EE were significantly different among groups. The highest values were recorded for R₄ ration of almost nutrients. Concentration of ammonia-nitrogen (NH₃-N) and total volatile fatty acids at 3 hrs post feeding were significantly higher in treated groups than that of control one. However, the least feed cost/kg gain was that of treatment (R4) followed by (R3) than the control. The biological treatments applied in the present study could be used without any adverse effect on the tested hemato–biochemical parameters. It could be concluded that replacement up to 30% of Berseem hay by biological treated corn cobs improved the productive performance of growing lambs.