A major physiological effect of cobalt deficiency is loss of appetite (Smith, 1997). Rumen micro-organisms require cobalt for the synthesis of Vitamin B12, which acts as a cofactor for protein and energy metabolism enzymes, namely methylmalonyl coenzyme A mutase and methionine synthase (Kennedy et, al. 1992). Cobalt deficiency, therefore, impairs the energy and protein metabolism and thus growth and development of the deficient animal, which can be defined as changes in the weight, shape and size of the body. Sheep tend to be extremely susceptible to Co deficiency and develop a normocytic and normochromic anaemia, anorexia, reduced weight gains and photosensitivity (Vellema et al., 1996). The present study was designed to evaluate the effects of dietary Co level on performance of Mehraban male lambs.