The effect of grape pomace (Vitis vinifera) and in oak leaf (Quercus libani) tannins by three runs of in vitro gas production on the organic matter digestibility (OMD) and in situ dry matter (DM) and crude protein (CP) disappearance were studied in four sheeps. Polyethylene glycol (PEG) was used to deactivate the tan-nins. In vitro gas production was recorded at 2, 4, 6, 8, 12, 48, 72, 96 and 120 h of incubation. The chemical composition (g/kg DM) of grape pomace and oak leaf were 940, 940 organic matter (OM); 94, 116 crude protein (CP); 568, 515 neutral detergent fiber (NDFom); 467, 316 acid detergent fiber (ADFom); 242, 93; (lignin (sa)); 70.5, 82 total phenols (TP); 49.7, 73 total tannins (TT); 79, 5.4 condensed tannin (CT) and 40, 70 hydrolysable tannin (HT). Using grape pomace and oak leaf decreased OMD, short chain fatty acids (SCFA), insoluble but fermentable fraction (b) and fermentation rate (c) comparing to control (P<0.05). The addition of PEG increased in vitro gas production (IVGP) at all times of incubation. Kinetics of gas produc-tion, OMD and SCFA were also increased by PEG incorporation (P<0.05). The increase in gas production (%) (IGP) in grape pomace (GP) diet was higher than those in oak leaf (OL) diet. The amounts of total pro-tozoa, Isotricha, Dasytricha, subfamily of Entodiniinae, Diplodiniinae and Ophrioscolecinae were de-creased by addition of grape pomace and oak leaf. The addition of PEG increased total protozoa, subfamily of Entodiniinae, Diplodiniinae and Ophrioscolecinae populations in grape pomace diet (P<0.05), but in-creased Isotricha, Dasytricha and subfamily of Diplodiniinae in oak leaf diet (P<0.05). The effective de-gradability (ED) (g/kg DM) of DM and CP for alfalfa, grape pomace and oak leaf were (646.6, 357.7 and 362.3) and (821, 227.3 and 202), respectively based on in situ fermentation. In conclusion, using grape po-mace and oak leaf have positively modified NH3-N concentration and protozoa population. Diet containing grape
