The purpose of this study was to determine the optimal ratio for improved dietary utilization by evaluating the effects of varying levels of mango seed kernel added to sheep diets on the nutritional value, digestion coefficients, and rumen fermentation. Four concentrate feed mixtures (CFM) were used to feed the Barki sheep: the first had a basal diet without mango seed kernels (MSK), the second had 5% MSK, the third had 10% MSK, and the fourth had 20% MSK. Using fully grown rams, the parameters of digestion were estimated, while rumen fermentation, the breakdown of dry and organic matters, and methane production using three ewes with rumen fistulas. The following are the outcomes: MSK diets had higher rates of nutrients digestion efficiency than the control diet which reflected on TDN values but not effect on DCP value. While, there was no statistically significant difference among MSK rations. The diet containing 10% MSK showed the higher retained nitrogen and its percentage in the diet; there were no statistically significant differences in the amount of nitrogen consumed between the diets. The pH values did not significantly differ among diets. The control diet yielded the higher rumen ammonia concentration, whereas the MSK diets produced the lowest. The third diet (10% MSK), which producing the most microbial protein, had the higher rate of ammonia and volatile fatty acid production. Adding MSK to rations increased the decomposition of the degradable fraction (b) in the rumen and the efficiency of dry matter decomposition (EDDM), and showed higher rates in those contained 10% MSK. The control ration showed a higher rate (c) of decomposition of crude protein in the rumen and thus the higher efficiency in decomposition protein (EDCP). In contrast, the non-degradable protein part (RUP) was higher in MSK containing diets. Methane production was significantly higher in the control diet than in diets containing MSK, and its percentage fell as the percentage of MSK in the diet increased. Addition of MSK at a rate of 5, 10 and 20%, resulted in a decrease in methane gas production by about 22.83, 30.32 and 39.95% respectively. Therefore, it was recommended to incorporate MSK in complementary diets for small ruminants at the rate of 10% with the possibility of increasing the rate to 20% without any negative effect on animal health