Abstract: Sixteen Simmental steers on corn straw diet were stratified by weight and randomly assigned in a block design experiment,which consisted of 0 mg (control), 8 mg, 16 mg or 24 mg of supplemental Cu (as copper methionine) per kg dry matter (DM) diet, to evaluate the effect of different copper methionine levels on nutrient digestibility, nitrogen balance and serum parameter. The results show that the digestibility of organic matter, crude protein, ether extract and nitrogen free extract in supplemented Cu treatments increases significantly (P < 0.05). The digestibility of neutral detergent and acid detergent fiber in groups supplemented with 8 mg and 16 mg Cu per kg DM diet are higher than those of the control (P < 0.05). Digestible nitrogen, retention nitrogen and the utilization of nitrogen in Cu treatments increase significantly (P < 0.05). The concentration of serum triglycerides, glucose, total cholesterol, albumin and total protein increases significantly (P < 0.05). Serum urine nitrogen concentration in groups supplemented with 24 mg Cu per kg DM diet decreases significantly (P < 0.05). The activity of serum glutamate oxaloacetate transaminase, glutamate pyruvate transaminase, alkaline phosphatase, acid phosphatase and lactate dehydrogenase is not significantly affected. The activity of serum ceruloplasmin increases significantly (P < 0.05). The activity of serum SOD and GSHpx as well increases significantly (P < 0.05). Serum maleic dialdehyde concentration decreases significantly (P < 0.05). Serum Cu concentration in groups supplemented 16 mg and 24 mg Cu per kg DM diet significantly increases over those of the control and 8 mg Cu per kg group (P < 0.05). Optimum Cu supplemented dose is 8 ～ 16 mg Cu per kg DM diet.