The phytochemical composition, antibacterial and antioxidant activities of methanolic extract produced from leaves of rosemary (Rosmarinus officinalis L.) which growing naturally in Libya were assessed. Rosemary extract showed superior scavenging activity in both of  Diphenyl-1-picrylhydrazyl (DPPH) (259.67 μmol TE/g dw) and azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) (228.04 μmol TE/g dw). Antimicrobial activity of rosemary against 10 food-borne pathogenic bacteria and food spoilage bacteria were determined and expressed as Minimum inhibitory concentrations (MIC) and Minimum bactericidal concentrations (MBC). Rosemary showed considerable antimicrobial activity against different test bacteria. Namely Bacillus cereus and Enterobacter feacaliswere found to be highly susceptible (MBC: ˂3.125 mg/ml) whereas Shigella sonnei and Staphylococcus aureus were the most resistant bacteria (MLC: > 25 mg/ml). Results for phytochemical screening of the methanolic extract of rosemary revealed that total content of phenol and total content of flavonoid were 29.23mg gallic acid equivalents/gm on a dry basis and 6.59 mg catechin equivalent/g on a dry basis respectively. Additionally, other components were detected consisted of saponin (35.40 mg/gm), tannins (32mg/gm) and alkaloids (119 mg/gm). Also, rosemary extract (1% aqueous solution) was examined for preservation of Seriola Dumeriri fish fillets. Rosemary extract reduced significantly the total viable, psychrotrophic and coliform counts on fish fillets stored at 2^oC and extended their shelf life up to 6 days in compared with the untrated samples.