A series of experiments were conducted to determinefatty acids, amino acids, protein contents and enzymes activities of the three algae (Ulva lactuca, Sargassum hornschuchii and Pterocladia capillacea), which were previously collected in the late of July 2009 from Abu Qir and subjected to UV-B radiation for 20, 40 & 60 minutes daily for five days. These parameters were estimated, when the UV-absorbing compounds contents recorded its maximum after the third day of irradiation of 60 minutes daily for the three algal species. This time was chosen as we expected a suitable response of the algal species to UV-B irradiation. Total saturated, mono unsaturated and polyunsaturated fatty acids of U. lactuca and S. hornschuchii were increased due to UV-B irradiation, the total fatty acids content in both irradiated algae increased also, while the contents of total saturated, mono and polyunsaturated fatty acids of Pterocladia capillacea decreased after exposure to UV-B radiation. The study shows that half of amino acids of Ulva lactuca increased after exposure to UV-B radiation for three days, while the other half of amino acids was decreased.S. hornschuchii showed the decreasing of all amino acids contents after exposure to UV-B radiation except the two basic amino acids histidine and lysine, and the aliphatic amino acid serine. All amino acids of P. capillacea increased after exposure to UV-B radiation for three days except the aliphatic amino acid serine and the aromatic amino acid tyrosine.The total protein content increased in U. lactuca and S. hornschuchii through out the irradiation experiment, while P. capillacea showed notable decreases of protein contents after UV-B irradiation. Exposure to UV-B radiation increased the activity of superoxide dismutase, ascorbate peroxidase and catalase of the three irradiated algal species. The findings also suggest that exposure to UV-B irradiance also affect the ultrastructure of all the irradiated algal species.