This study was conducted with the objective of evaluating the nutritional and production aspects of partially replacing berseem hay (BH) with cassava (Manihot esculenta) hay (CMH) “leaves and thin twigs" in Barki ewes' diets in terms of ruminal fermentation, milk production & it's composition, some serum characteristics and lambs' performance. Forty late pregnant Barki ewes with an average live body weight of 53.40±4.20 Kg were assigned to four experimental (n=10 each) diets, i.e. Control [40% berseem hay “BH" plus 60% Concentrate Feed Mixture “CFM"] or [25, 50 and 75% replacement of BH by CMH] plus the CFM for a period started 4 weeks before the expected lambing date and lasted post lambing until 8 weeks of lactation. The results showed that replacement of 75% BH with CMH resulted in increasing (P<0.05) the fractional volatile fatty acids; VFA (Acetate and propionate) and total VFA concentrations compared to other diets. Ruminal NH₃-N concentrations and total protozoa were decreased (P<0.05) with 50 and 75% CMH replacement compared with the control. Milk yield as well as milk fat, protein and lactose percentages were improved (P<0.05) by replacing 75% of BH with CMH rather than other treatments. Serum total protein, albumin, globulin and urea-N slightly decreased in CMH groups than those in the BH group, while creatinine and liver enzymes (AST & ALT) were slightly increased. Also, the level of 75% replacement of BH with CMH significantly improved lambs growth rate and weaning weight compared with other diets. Thus, using 75% CMH as a replacement of BH provides a promising source of forages for sheep with positive impacts on rumen fermentation and ewes' productivity.