Abstract: Crude oil degrading bacteria play very important role in the biodegradation of environmental petroleum contaminants. In this study, many petroleum-hydrocarbon degrading bacteria were isolated from the crude oil-contaminated seawater, oil sludge, as well as oil-water and sludge mixture in Bohai bay. The strains were identified on the basis of their physiological, biochemical characteristics and 16SrRNA gene sequence analysis, and their degradation capacities of alkanes, cycloalkanes, and aromatics were evaluated by the GC-MS method. The three bacterial strains that possess high petroleum hydrocarbon degrading capabilities were isolated. The three bacteria strains showed 99% similarity with Acinetobacter by 16SrRNA sequence, and these strains were preliminarily identified as Acinetobacter species. The three bacterial strains have exhibited obvious petroleum hydrocarbon degrading capabilities, and the best performer is the strain Acinetobacter sp. Tust-DM21, followed by the stain Acinetobacter sp. Tust-DC12, and then is the strain Acinetobacter sp. Tust-DW04. Biodegradation rate of crude oil compositions were observed as alkanes > aromatics > cycloalkanes. Among the three strains, the strain Acinetobacter sp. Tust-DM21 is the most effective petroleum hydrocarbon degrading bacterium. The strain shows degradation rate of alkanes (C10~C30) reaching up to 98%, and degradation rate of aromatics and cycloalkanes reaching up to 88% during 10 days of incubation at 28℃. These results suggest that Acinetobacter sp. Tust-DM21 has highly effective degrading capability related to alkanes, aromatics and cycloalkanes. Therefore, Acinetobacter sp. Tust-DM21 may employ as an excellent degrader in the future for the bioremediation of marine environments polluted by crude oil.