THE CURRENT study aimed to investigate the effects of nine probiotic bacterial strains with different levels of supplementation in a highly concentrated diet in sheep on ruminal fermentation parameters in in vitro. the examined bacterial strains were lactobacillus cassia (LC), lactobacillus plantarum (LP), lactobacillus acidophilus (LA), lactobacillus bulgaricus (LB), bacillus subtillus (BS), bacillus lichnoformis (BL), bifidobucterium bifidum (BB), enterococcus faecium (EF), and clostridium butyricum (CB). the probiotics were tested at 0 (control), 0.25, 0.5, 1, 2, and 4×109 cfu/g feed. The gas production value decreased by LA and LP strains, while BL, EF, BB, and CB strains increased gas production. methane production was reduced by LC, LP, and BS strains, whereas it increased by BL and EF strains. BS, CB, and LP strains improved dry matter degradability (DMD), while LC, LB, and BB strains resulted in lower values. organic matter degradability was enhanced by the addition of strains such as BL, CB, LA, and LB. certain strains, including LA, LC, LP, and LB, reduced NH3-N production, while EF supplementation increased NH3-N levels. total volatile fatty acid production was generally enhanced by the addition of bacterial strains, except for LA and BS strains, which showed lower production. pH values were influenced by bacterial strains. the LC and LP exhibited the lowest pH, while the CB, LB, and LA strains had the highest pH values. in conclusion, the best strain was LP which reduced methane and NH3-N production and improved DMD. The best improvement occurred with the high levels of addition.