In vitro, studies were carried out to investigate the effect of using yeast (Y), fibrolytic enzymes (FEN) and their mixture (Y+FEN ratio 1:1) on rumen fermentation using rumen fluid. Three levels of each additive were used (1, 2, and 3 g/kg diet). At each level, a sample (300 ±5 mg) of the contained clover hay (40%) and concentrate mixture (60%) was weighed into 125 mL glass bottles (six bottles per treatment) and two blank bottles. Each of these bottles was filled up with 40 ml of a mixture of rumen fluid and buffer solution (1:3 v/v). After 24 hours of incubation at 39ᵒC, in vitro, total gas production (GP), dry-matter disappearance (IVDMD), organic-matter disappearance (IVOMD), and CO2 were recorded. The results showed that by adding 2g of FEN, 3 g of Y, and 3g of Y+FEN, the concentration of Short chain fatty acids (SCFA), Ammonia (NH3-N), CO2, and GP levels increased significantly (P<0.05). Treatment 3g (Y+FEN) recorded the highest values of SCFA (1.46 mmol/g DM), NH3-N (6.96 mg/dl), and gas production (123 ml/g). The highest concentration of CO2 was detected at Y(3g), FEN(2g), and Y+FEN (1g) (67.46, 67.85, and 68.16), respectively. Significant (P<0.05) increase in the digestibility of NDF, ADF, ADL, hemicellulose, cellulose, DM, OM, CP, and CF of treatments FEN (2g) and Y (3g) and Y+FEN (2g). It is recommended to utilize yeast (3 g/kg diet) and fibrolytic enzyme (2 g/kg diet) or their mixture (1:1) at 2 g/kg diet in ruminant animal feed to create favorable rumen conditions.