Abstract: Effects of three petroleum hydrocarbon solutions(marine fuel L.D.O.,Saudi Arabian light crude oil and marine lubricating oil)at growth and photosynthesis in sea weed Ulva pertusa were studied to provide basic data for oil spill monitoring and risk assessment.The concentration of L.D.O.and Saudi Arabian light crude oil petroleum hydrocarbon was 0,5,10,15,25,and 50 mg/L and of marine lubricating petroleum hydrocarbon was 0,5, 10,20,25,50,and 100 mg/L.The result showed that the sea weed was found to survive in three oil solutions under low concentration,but high oil concentration(L.D.O.=50 mg/L;Saudi Arabian light crude oil=50 mg/L; marine lubricating oil=100 mg/L)to be dead totally within 96 h.The Chlorophyll contents,photosynthetic rate and respiratory rate were decreased in the sea weed exposed to the elevated concentrations of three petroleum hydrocarbon solutions,and the Chlorophyll contents and photosynthetic rate of the sea weed was shown the tendency of undulate.However,the photosynthetic rate was decreased in the sea weed in marine lubricating oil petroleum hydrocarbon solutions with increasing oil concentration.