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 gastrointestinal tract (McGuffey et al., 2001). In Ruminants, monensin has shown selective inhibition of ruminal bacteria, lactic and methane producers and stimulation of propionate production by 25% (Matabudul et al., 2001). In lambs, monensin improves weight gain and feed conversion ratio (FCR) and depressed the dry matter intake (DMI) (Martini et al., 1996), but lasalocid improve average daily gain (ADG) without depressed the DMI (Swanson et al., 2000). There is limited information about the effect of ionophores on performance of Iranian fattening lambs and particularly no works was done with Arabi lambs. Therefore, the present study was conducted to investigate the effects of feeding monensin and lasalocid on performance and carcass characteristics of fattening Arabi lambs.