A couple of experiments were conducted to study the effects of different levels of dietary cobalt (Co) on performance, concentration of plasma vitamin ¬B¬12 and microbial protein synthesis in Mehraban male lambs. In the first experiment 28 lambs were randomly divided into four groups and fed by a basal diet containing 0.088 mg Co/kg of Dry Matter (DM) supplemented with 0 (control), 0.25, 0.50, or 1.00 mg Co/kg DM for a duration of 70 days. Lambs were weighted in days 0, 35 and 70 of the experiment with daily feed intake recorded to determine gain efficiency. Blood samples were collected to determine plasma vitamin¬ B¬12 on day 1 and 68. As for the second experiment, four lambs from each treatment of the first experiment were randomly allotted to the individual metabolic crates to assess the effects of dietary Co on microbial protein synthesis. This trial lasted for 12 days of a 6-day adaptation period and subsequent 6-day collection period for recording urine excretion vs. feed intake. Average daily gain, average daily feed intake and gain efficiency were improved by Co supplementation. The average daily gain and gain efficiency was recorded as the highest in treatment 3 (P < 0.05). Plasma vitamin¬ B¬12 on day 68 was significantly lower in treatment 1 (P < 0.05). Excretion of purine derivatives, purine absorption, microbial nitrogen entering the duodenum, microbial nitrogen per kilogram of Digestible Organic Matter Intake (DOMI) and microbial nitrogen per kilogram of Digested Organic Matter in the rumen (DOMR) were increased (P < 0.05) through Co supplementation. It was finally concluded that Co supplementation at a level of 0.5 mg of Co/Kg of DM in a basal diet containing .088 mg of Co/kg DM is the optimum level for enhancing performance, vitamin B12 production and microbial protein synthesis in Mehraban male lambs.