Naphthenic acids (NAs) as toxic components in most petroleum sources are suspected to be one of the major pollutants in the aquatic environment following oil spills, and the polarity and persistence of NAs make it a potential indicator for oil contamination. However, the contamination and potential effects of pollutants in oil spill affected areas remain unknown. To investigate NAs in oil spill affected areas, a sensitive method was first established for analysis of NAs, together with oxy-NAs in sediment samples by UPLC-QTOF-MS. Then the method was applied to determine the NA mixtures in crude oil, weathered oil, and sediments from the spilled sites after the Hebei Spirit oil spill, Taean, South Korea (Dec. 2007). Concentrations of NAs, O3-NAs, and O4-NAs were found to be 7.8-130, 3.6-44, and 0.8-20 mg kg(-1) dw in sediments from the Taean area, respectively, which were much greater than those measured in the reference sites of Manlipo and Anmyundo beaches. Concentrations of NAs were 50-100 times greater than those (0.077-2.5 mg kg(-1) dw) of PAHs in the same sediment samples, thus the ecological risk of NAs in oil spill affected areas deserves more attention. The sedimentary profiles of oil-derived NAs and background NAs centered around compounds with 21-35 and 12-21 carbons, respectively, indicating that the crude-derived NA mixtures originating from the 2007 oil spill were persistent. Acyclic NAsn=5-20 were easily degraded compared to cyclic NAsn=21-41 during the oil weathering processes, and the ratio of oxy-NAsn=21-41 relative to NAsn=21-41 could be a novel index to estimate the degree of oil weathering in sediments. Altogether, the persistent oil-derived NAsn=21-41 could be used as a potential indicator for oil-specific contamination, as such compounds would not be much affected by the properties of coastal sediments possibly due to the high sorption of the negatively charged compounds (NAs) in sediment.