This study was conducted at the farm of Ruminants Research Station, Office of Agricultural Research, Ministry of Agriculture, Abu Ghraib – Baghdad. The experiment continued for 56 days, in addition to 14 days as preliminary period from 11/10/2015 until 06/12/2016 to study the effect of treating barley straw with baker’s yeast (Saccharomyces cerevisiae) on some productive performance of growing lambs. Ten Awassi lambs used in this experiment with average initial weight of 38±0.50 kg and age 8-9 months. Lambs divided randomly into two equal groups and kept in separate individual pens. Lambs fed individually on a unified concentrate diet (2.5% of body weight), while, roughages offered ad libitum. Each group fed on roughages (barley straw), as follows: 1. The first group fed untreated barley straw plus concentrate mixture and operated as control 2. The second group fed barley straw treated with baker’s yeast (Saccharomyces cerevisiae) at rate 0.5% (5 kg / tones). Animals weighed every two weeks before morning feeding. The digestibility experiment performed in the seventh week. Rumen liquor collected at fifth week of the experiment at three times (before feeding, after three and six hours from feeding). Results showed different effects among treatments. The yeast not significantly affected the feed intake, average daily gain (15 – 28 d), in vivo digestibility, and rumen fermentation (pH and NH3-N). Moreover, there was significant decrease (P <0.05) in daily weight gain during 29 - 42 d with baker’s yeast compared with control (107.14 and 160.71g/d, respectively), while a significant increase (P <0.05) found in daily weight gain in the (1 – 14 d and 43 – 56 d) with baker’s yeast (103.57 and 178.57 g/d, respectively) compared with control (48.33 and 142.86 g/d, respectively). We conclude that treatment with baker’s yeast not improved feed conversion ratio, feed intake, intake of different nutrients, weight gains, digestibility and rumen fermentation (pH and NH3-N concentration).