In compliance to the recent surveys on algal species and their potentials to produce biologically active natural products, collection of algae were carried out during summer 2014 from five water habitats in Siwa Oasis. Three common microalgal pure isolates including one green alga Chlorella vulgaris and twoblue green algae Chroococcus turgidus and Phormidium tenue were obtained using BG11, Basal Bold and Zarrouk media. The methanolic extracts of the three algal species were tested for the antibacterial activity against eight human pathogenic bacteria Enterococcus sp., Staphylococcus aureus, Acinetobacter baumannii, Escherichia coli, Klebsiella pneumonia, Proteus sp., Pseudomonas aeruginosa and Salmonella tyhphi using the agar well diffusion method. The results revealed that Phormidium tenue showed the highest promising antibacterial activity so it was choosed for successive extraction by using petroleum ether, ethylacetate, 80%methanol and water solvents. Among the tested solvents the ethyl acetate successive extract showed the highest antibacterial activity. Results of some chromatographic investigations for the ethyl acetate extract indicated that the phenolic compounds were the main components. High performance liquid chromatography (HPLC) technique was used for qualitative and quantitative analysis of the bioactive phenolic compounds. It resulted in identification of 22 phenolic compounds with different concentrations ranging from 1755.2 to10.425 μgg^-1for ethyl vanillate and catechol compounds, respectively.