Abstract: Seventy Mehraban male lambs (initial live weight 43.9±4.3 kg) were used in a 70-day feeding experiment. Lambs were fed with 12 diets in a completely randomized design arranged in a 3x4 factorial trial with three levels of thiamin (0, 4 and 6 mg kg-1 DM) and 4 levels of monensin (0, 5, 11 and 22 mg kg-1 DM). A 21-day period was included for adaptation to the diets. Basal diet (dry matter basis) consisted of 7.5% corn silage, 8% alfalfa hay, 70% barley grain, 10% wheat bran, 3% cottonseed meal, 1.1% limestone and 0.4% vitamin and mineral supplement. Carcass characteristics, average daily gain and Feed Conversion Ratio (FCR) were not significantly (p>0.05) different between diets containing monensin and thiamin or their combinations with control. Daily dry matter intake was lowest (p<0.05) for diets 8 (11 mg monensin and 4 mg thiamin per kg DM) and 12 (22 mg monensin and 6 mg thiamin per kg DM) compared with diet 3 (6 mg thiamin per kg DM). BUN level and ruminal fluid pH were not significantly (p>0.05) different between diets. Blood glucose of lambs fed with 11 mg monensin/kg DM (50.6 mg dL-1) was higher (p<0.05) than control group (45.8 mg dL-1). Monensin tended to improve FCR. Monensin reduced DMI (p<0.05) and decreased feed consumption by 9.13-9.75% compared with the control diet. The effect of monensin on blood glucose concentration was significant (p<0.05) which was higher for two levels of monensin (11 and 22 mg kg-1 DM), compared with the control diet. Ruminal fluid pH was significantly (p<0.05) higher with all levels of monensin and at 22 mg kg-1 monensin, ruminal ammonia concentration was lowest (p<0.05). The overall effect of thiamin was a decrease (p<0.05) in BUN concentration.