Pomegranate peels (PP) and mango leaves (ML) were analysed for nutrients and tannin contents. In an in vitro test, ten diets were prepared; six contained 2, 4 and 6% of PP or ML, three diets supplemented with mixed levels of PP and ML (1 + 1%, 2 + 2% and 3 + 3%) and control diet free of them. Gas was measured after 3, 6, 12, 24, 48 and 72 h of incubation. Methane and rumen parameters were estimated. In an in vivo experiment, 40 Ossimi lambs were divided into four groups; the first was control, other groups (T1, T2 and T3) fed diets containing 6% PP, 6% ML or mix levels (3% PP + 3%ML), respectively, for 2 months. Results showed that PP and ML were rich in tannins. In the in vitro test, a maximum reduction in gas, methane and NH3-N was in 6% PP, 6% ML and mixed levels (3% PP + 3% ML). In the in vivo experiment, there were no differences in growth and digestibility of DM and nutrients between treatment groups. Only a lowered DM intake and protein digestibility in lambs fed 6% PP. Gas and methane emission was decreased significantly in lambs fed 6% PP, compared to other groups. TVFAs and NH3-N were decreased for treatment groups. Also, all treatments did not show any pathological changes in liver function or on oxidative stress markers. In conclusion, PP and ML can be used in sheep diets at inclusion levels of 6% and mixture without detrimental effects on general health of Ossimi lambs.