The major sources of PAHs are anthropogenic. These may be generated by pyrolytic and petrogenic processes. Low molecular weight PAHs come into the aquatic system through the accidentally leakage of oil spills. Incomplete combustion at high temperature will release high molecular weight PAHs by products of pyrolytic processes. The methods for assigning sources of PAHs include paired isomeric ratios. In this study, the ratios of Low Molecular Weight/High Molecular Weight, Phenanthrene /Anthracene, Fluoranthene/pyrene and Benzo(a)anthracene/Chrysene were used to recognize the sources of PAHs. So in October 2009, 3 locations were selected from Mousa Inlet in the Persian Gulf (Jafari, Ghanam and Ahmadi). Sampling of sea water, phytoplanktons, zooplanktons, and two types of fish, mullet (Liza abu), and sea bream (Acanthopagrus latus) were done. Samples were analysed by High Performance Liquid chromatography (HPLC) equipped with UV detector. The ratios showed that PAHs in this area originated from both pyrolytic and petrogenic inputs, predominantly from pyrolytic origin.