Pomegranate peel extracts (PPE) have been found to exert beneficial effects on animal health due to their antioxidant, antibacterial and immunological effects. Sainfoin hay that contains moderate to high content of condensed tannins, was reported the reduced rumen proteolysis and decreased urine N excretion in beef cattle. The objective of this study was to determine the effect of sainfoin hay extract (SHE) and PPE supplementation on in vitro rumen fermentation and protein metabolism of high-protein finishing diet using rumen simulation technique (Rusitec). The experiment was a completely randomized design with 3 treatments: Control (10% barley silage, 60% barley grain and 30% wheat distillers grain), and control supplemented with SHE (4.1 g catechin equivalent/kg DM) or with PPE (2.8 g tannic acid equivalents/kg DM). The experiment consisted of 10 d of adaptation and 7 d of data collection. Concentration of volatile fatty acid (VFA) was lower (44.1 or 41.6 vs. 48.2 mM; P < 0.02) but molar proportion of acetate was higher (37.2 or 37.4 vs. 34.9%; P < 0.01) with SHE or PPE vs. control. Branched-chain VFA decreased (P < 0.01) with PPE (0.9%) compared with control. Concentrations (mg/100 mL) of large (10.1) and small peptides (6.1) were not affected but that of NH₃-N was lower (P < 0.01) with PPE (6.3) vs. control (8.1). Digestibility of DM, starch and crude protein decreased (P < 0.02) with SHE (58.7, 74.7 and 43.0%, respectively) and PPE (55.5, 68.9 and 43.4%, respectively) supplementation compared with control (63.3, 83.2 and 51.0%, respectively). Bacterial N production (averaged 52 mg/d) and bacterial efficiency (averaged 10.3 g bacterial N/kg digested OM) were not affected by SHE or PPE supplementation. Overall, there were no differences in VFA concentrations, N fractions and nutrient digestibility between SHE and PPE except for the concentrations of branched-chain VFA and NH₃-N which were higher (P < 0.01) with SHE versus PPE supplementation. These results indicated that supplementation of high-grain diet with SHE or PPE increased rumen by-pass protein; however, suppression of feed fermentation may decrease the feeding value of high-grain diet.