Pneumonia is a common disease in sheep flocks that causes severe economic losses and high mortality rate among sheep due to high resistance to ordinary standard antimicrobial treatment protocols. This study was to assess silymarin efficacy as an antibacterial agent against some isolated bacterial strains from twenty-five pneumonic sheep in vitro. Nasopharyngeal swabs were collected from sick sheep, and placed in a nutrient and pleuro pneumonia like organism broth (PPLO) for bacterial isolation. P. aeruginosa, S. aureus, E. coli, and M. ovipeumoniae were isolated bacterial species confirmed by PCR using specific genes. The antibacterial activity of silymarin against P. aeruginosa, S. aureus, and E. coli was evaluated using the well-diffusion technique. It was shown that the minimum therapeutic dosage of silymarin was not less than 280 mg/ml against P. aeruginosa, whereas chloramphenicol had little effect. Chloramphenicol exhibited more antibacterial activity against S. aureus and, E. coli compared to different concentrations of silymarin. The microbroth dilution method determined the minimum inhibitory concentration (MIC) of silymarin against P. aeruginosa, S. aureus, E. coli, and M. ovipeumoniae were 2.14, 0.39, 0.38, and 2.5 mg/ml, respectively. The Minimum Bactericidal Concentrations (MBC) of silymarin were verified by the absence of bacterial growth of the isolated strains that were scattered from the lowest MIC. In conclusion, silymarin exhibited antibacterial efficacy against isolated P. aeruginosa, S. aureus, E. coli, and M. ovipeumoniae from pneumonic sheep in vitro compared to chloramphenicol, suggesting its therapeutic value in sheep.