Strain P127 was isolated from the overlying-water of sediment sampled at 3542 m water depth from Atlantic Ocean, after enrichment using a PAH mixture (naphthalene, phenanthrene, anthracene and pyrene) as the sole carbon source. A systematic characterization including cellular fatty acid composition, G+C content, and 16S rRNA sequence supports that this isolate is affiliated to Alteromonas macleodii. In The phylogenetic tree of gyrB gene, strain P127 was grouped with the surface-ecotype strains instead of deep-ecotype. Strain P127 was able to use naphthalene, 2-methylnaphthalene, phenanthrene, pyrene, genetic acid as a sole carbon and energy source. It could transform dibenzofuran and 4-methyl-dibenzothiophene quickly accompanied by color change of medium to bright orange. A fragment of PAH initial dioxygenase iron sulfur protein (ISP) gene was PCR amplified from strain PI27 by using degenerate primers. It showed high identity (>90％) with the dioxygenase from Pseudoalteromonas sp. EH-2-1, Neptunomonas naphthovorans strain NAG-2N-113 and NAG-2N-126, while less than 60％ similarity with the genes from other strains. Strain PI27 is the first bacterium of genus Alteromonas as a PAH degrader. It may prove a useful candidate for bioremediation of marine environment.