In some part of world for intensive production of fattening of lambs, they are fed with high amount of concentrate. However this pattern of feeding often affect negatively of rumen fermentation (Mould et al 1983). Ionophores act by interrupting transmembrane movement and intracellular equilibrium of ions in certain classes of bacteria and protozoa that inhibit the gastro intestinal tract (McGuffey et al 2001).The ionophore monensin can improve cellulose digestion of diets high in readily available carbohydrate by inhibition the growth of lactate-producing bacteria (Russell and Stroble 1989). An alternative method to reduce these negative effects could be the using step-up feeding by supplementation of monensin. The objective of this study was to obtain information on animal performance receiving diets high concentrate: forage ratio with different level of monensin.