The purpose of this study is to investigate and evaluate the antibacterial activities of Rosmarinusofficinalis extracts (ethanol/water) against four strains of bacteria: Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853), Staphylococcus aureus (ATCC 25923) and Bacillus Cereus (ATCC 11778) using the diffusion method. The results revealed that all extracts have a certain biological activity against gram-negative and Gram-positive bacteria at 1000 and 5000 g/ml. In addition, the extracts (ethanol/water) showed the highest activity, is against Pseudomonas aeruginosa. The combination of Rosmarinusofficinalis with each of the standard antibiotics: (SP25 μg) (Spiramycin), (MT5μg) (Metromidazole), AMX (amoxicillin), CZN (cefazolin), cxn (Cefalexin). They were more active and showed significant synergistic effects, extracts (ethanol/water) of medicinal plants Rosmarinusofficinalis, It also showed strong synergistic effects. The results of this study suggest that Rosmarinusofficinalis could be used to treat diseases caused by the organisms studied. Further, pharmacological and chemical studies can be carried out in order to isolate and identify the chemical constituents responsible for antimicrobial activity in selected plants.