Abstract: In order to identify the oil source, depositional environment, thermal maturity and possible source rocks of the oil in eastern Wushi sag of Beibu Gulf basin, chromatography and mess-spectrometric techniques are used to systematically analyze aromatic hydrocarbon compositions in oil samples. The results show that aromatic hydrocarbon in the oil of eastern Wushi sag is characterized by high content of aphthalene and phenanthrene series, low content of biphenyl, threefluorene and triaromaticsterane series. Compositions such as 1,2,5,6 methyl naphthalene, retene and triaromatic sterane indicate that the organic matters in the source rocks are of double contributions from both aquatic organisms and terrestrial higher plants and form in reduction and weak reduction-weak oxidation environments of fresh water-saline water lacustrine facies. Vitrinite reflectances obtained from methyldibenzothiophenes ratio and distribution index, methyl naphthalene ratio, methylphenanthrene index and distribution fraction respectively show that the crude oil belongs to moderate maturity oil. Based on the compositions of aromatic hydrocarbons in the oil, the oil in the study area can be divided into 3 subgroups of 2 types, which includes type Ⅰ oil originated from the source rocks of the upper and lower El2; type Ⅱ1 oil from the lower El2 and type Ⅱ2 oil from the lower El2 and possibly El3