en Biotechnology Biotechnology پژوهشي Orginal research papers Due to numerous reasons, marine-obtained resources have attracted the interest of researchers. One of the subjects attracted the interest of researchers in recent years in the field of biologist marine algae due to their nutritional value, their benefits for health and their biological activities. In this study, the powder was extracted using acetone, then the extract purified by silica gel column chromatography with n-hexane and ethyl acetate combination to isolate the steroid compounds. Isolated compound was run through TLC and sprayed with vanillin-sulphuric acid reagent to detect the steroid compounds and profile of isolated compounds obtained by gas chromatography (GC). Antibacterial activity of steroids extracted of <a href="https://www.google.com/search?rlz=1C1GCEA_enIR825IR825&sxsrf=ACYBGNQZSvzK_q85LPMegGhR6AeYTRVApw:1573463782470&q=Ulvaceae&stick=H4sIAAAAAAAAAOPgE-LUz9U3MEnOK6xUAjPNjEzN4rUss5Ot9JMy83Py0yv184vSE_Myi3Pjk3MSi4sz0zKTE0sy8_OsMjLTM1KLFFBFF7FyhOaUJSanJqYCAJQuXT9eAAAA&sa=X&ved=2ahUKEwjWtsiu6eHlAhVCa1AKHTRqDRMQmxMoATAbegQIDhAU">green</a> algae <em>Ulva fasciata</em> from Qeshm Island in the Persian Gulf were assessed (<em>in vitro</em>). The antibacterial activity of the fractions was determined by Broth Dilution Methods against clinical Gram-negative bacteria: Escherichia coli, <em><em>Pseudomonas aeruginosa</em></em>, <em>Salmonella. typhi</em> and Gram-positive bacteria: <em><em>Staphylococcus </em></em><em><em>aureus</em>, </em><em><em>Bacillus subtilis</em></em>, Bacillus cereus. The steroids; cholest-7-en-3ol and cholestan-3-one, cyclic 1, 2-ethanediyl acetal compounds in column fraction were identified by GC. The results showed the minimum bactericidal concentration (MBC) of the cholest-7-en-3ol and cholestan-3-one, cyclic 1,2-ethanediyl acetal for S. aureus and Bacillus cereus were 500 &mu;g ml^-1, the MBC of the cholest-7-en-3ol for B. subtilis was 500 &mu;g ml^-1 and 1000 &mu;g ml^-1detected for cholestan-3-one, cyclic 1,2-ethanediyl acetal. The compounds had neither antibacterial activity on gram negative bacteria. Based on our results, extracted steroids from <em>U. fasciata</em> can be considered as a source of novel antibiotics. Green algae, Ulva fasciata, Antibacterial activities, Natural products, Persian Gulf 2384 2393 http://jifro.ir/browse.php?a_code=A-10-3512-1&slc_lang=en&sid=1 R. Azizi 100319475328460036373 100319475328460036373 No Department of Hygiene, Science and Research Branch, Islamic Azad University, Tehran A.A. Motallebi Moghanjoughi motalebi@ifro.ir 100319475328460036374 100319475328460036374 Yes Department of Hygiene, Science and Research Branch, Islamic Azad University, Tehran M. Nazemi 100319475328460036375 100319475328460036375 No Persian Gulf and Oman Sea Ecological Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Bandar Abbas, Iran M. Sharif Ruhani 100319475328460036376 100319475328460036376 No Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extention (AREEO) M. Afshar Nasab 100319475328460036377 100319475328460036377 No Kherad higher education center٫ Bushehr, Iran.