Abstract: Green algae are promising source of various types of biofuels and many are capable to catalyze production of hydrogen. In the present work isolation and characterization of green alga collected from Jordan’s coastal part of Gulf of Aqaba are reported. Morphological details of the purified green alga GAF99 indicated it belongs to genus Chaetomorpha. Potential of the alga for hydrogen production in sea water bioreactor was investigated under various environmental conditions. Hydrogen productivity of Chaetomorpha sp. GAF99 was influenced by temperature and light. The main algal capability of hydrogen production occurs during dark period at 23±1°C and it is decreased drastically at higher temperature. Gas chromatography analysis of gaseous mixture obtained under different environmental conditions showed that the percentages of hydrogen gas were in the range 97.767 -99.494. The reported results might indicate that green alga GAF99 has a promising potential for hydrogen production under specific environmental conditions.