This study aims to investigate the possibility of using marine microalgae (green microalgae) to degrade petroleum hydrocarbons and converse them into lipids in cells. Rapid growth strains of Chlorella sp. and Dunaliella salina were obtained by single separation technology and ultraviolet mutation breeding techniques. The mutagenesis dominant strains were then cultured in oily seawater with different oil concentrations. The contents of conjugated hydrocarbon compounds and petroleum aromatic hydrocarbons were determined respectively by UV spectrophotometry and fluorescence spectrophotometry. The lipid content of the microalgae cells was measured by the Soxhlet extraction method. The results showed that Chlorella and D. salina were able to degrade petroleum hydrocarbons effectively at oil concentrations of 1.5~10.0μg/ml. The degradation rates of conjugated hydrocarbon compounds were 25.3%~39.5% and 17.9%~24.0%, respectively, while the degradation rates of petroleum aromatic hydrocarbons were 18.7%~26.2% and 22.1%~30.2%, respectively. Degradation rates for both petroleum hydrocarbons were the highest at oil concentration of 3.5μg/ml, and the performance of Chlorella was slightly better than D. salina. The lipid contents of the algal cells were 1.2 and 1.5 folds of the control when cultured in oily seawater at oil concentrations of 1.5μg/ml and 3.5μg/ml, and the lipid content of the algal cells was the highest for the latter.