This study was conducted at Al-Rahep farm and the nutrition laboratory at the Animal Production Department, Faculty of Agriculture, Menoufia University (Shebin El-Kom), to investigate the effect of yeast extract on the degradation of low-quality roughage, digestibility, rumen fermentation, blood criteria, and immunity status of Barki sheep. Commercial dry yeast products (EGYBELG®, LA ROYALE Instant dry yeast, manufactured by Egyptian Belgian Co. Egypt) prepared for yeast extract (YE). Viable yeast cell concentration was 2.44 × 1011 CFU/g yeast product. The yeast extract was prepared and checked for enzyme activity. Tested grounded roughages, including rice straw (RS), corn cops (CC), corn stover (CS), and wheat straw (WS) supplemented with three levels of YE; the 1st: without yeast extract (YE0), 2 g yeast extract /kg DM (YEL1) and 4 g yeast extract /kg DM (YEL2) were used in an in-vitro digestibility trial. The in-vitro study revealed that RS supplemented with yeast extract at 4g/kg DM (YEL2) recorded the highest in-vitro dry matter, organic matter, and crude protein digestibility (IVDMD, IVOMD, and IVCPD) values. To clarify the results of the in-vitro study, a digestibility trial was designed on 15 Barki rams (five rams/group). Three rations were prepared with the same roughage: concentrate ratio of 30:70%; Ration 1 (RSR):70% concentrate feed mixture (CFM) + 30% rice straw (control negative). Ration 2 (YER): control negative ration supplemented with 4 g YE/kg DM. Ration 3 (CHR): 70%CFM + 30% clover hay (control positive).