Crude protein content in animal diet is considered the first important factor for nutritionists when they start to formulate a diet for different types of animals. Therefore, this experiment was conducted to investigate the effect of substituting soybean seed meal by different protein sources on lamb’s diet in terms of nutrients digestibility, and growth performance. Therefore, four tested concentrate feed mixtures (CFM, 14% crude protein) were designed. The CFM1 contained soybean meal as the only source of protein, and 50% of soybean meal protein was replaced by either sunflower meal, sesame seed meal, or black seed meal in CFM2, CFM3, and CFM4 groups, respectively. A total of 28 growing Barki male lambs aged 6 months with an average body weight of 38.6 ± 0.4 Kg were used in feeding trials for 120 days. Depending on their body weight, lambs were divided into four groups (7 animals in each). Each group fed on one of the experimental diets, including D1 (CFM1+ clover hay), D2 (CFM2 + clover hay), D3 (CFM3 + clover hay), and D4 (CFM4 + clover hay). Results of digestion coefficients indicted that the digestibility of crude protein and the nitrogen-free extract was significantly higher for D1 and D4, compared to D2 and D3. The same trend was observed in nutritive values expressed as total digestible nutrients (TDN) and digestible crude protein (DCP), whereas the D1 and D4 recorded higher TDN and DCP, compared to D2 and D3. Results of rumen parameters demonstrated that there were insignificant differences among groups regarding the average pH, ammonia (NH3-N), and total volatile fatty acids (TVFAs). Data of growth performance indicated that the final live body weight, total body gain, and average daily gain of lambs feed D4 (56.75 kg, 18.62 kg, and 155 g, respectively) and D1 (56.62 kg, 17.60 kg, and 147 g, respectively) were higher than those for lambs fed D2 (53.40 Kg, 14.64 kg, and 122 g, respectively) and D3 (53.11 kg, 14.61 kg, and 122 g, respectively). The feed conversion ratio (feed/gain) was recorded better value with lambs fed D4 followed by that fed D1. In conclusion, black seed meal could be replaced with the 50% of soybean meal participation of protein in growing Barki lambs’ diets without any adverse effects on digestibility, rumen kinetics, and growth performance.