In this study. the effects of diets containg 10% straw (as source of crude fiber) + 90% concentrate (with straw group) and 2. 5 % NaHC0₃ (as source ot buffer) + 97. 5% concentrate (with NaHC0₃ group) on leedlot performance by 24 lambs (average initial weight 36- 37 kg 9 months of age) were evaluated. At the end of study, feed intake was lower in the with NaHC0₃ group. Average daily gain and feed conversion ratio were higher in the with NaHC0₃ group than straw group . The increasing of daily gain and decreasing of feed intake were significant statistically. Although, carcass weights were higher in with NaHC0₃ group than with straw group but this differences were not significantly statistically . There were no differences beetwen groups in another slaughter and carcass characteristics.