Geochemical studies on thirty-one（31）crude oil samples from the Muglad Basin,Sudan have been performed to investigate the influence of biodegradation on the abundance and distribution of polycyclic aromatic hydrocarbons（PAHs）.In order to confidently assert biodegradation as the primary control on the change in relative abundance and distribution pattern of crude oils of Muglad Basin,it was demonstrated that the crude oils from this basin belong to one single oil family and originated from a common source rock of Abu Gabra Formation.Furthermore,the effective maturity parameters from C31 hopane 22S/（22S + 22R）,C29 sterane 20S/（20S + 20R）indicate that the analyzed crude oils samples derive from source rock in the main oil generation window.This study argues the changes in relative abundance and distributions patterns of aromatic hydrocarbons in crude oils of the Muglad Basin likely reflect microbial biodegradation and rule out the potential effects of origin and thermal maturity of the oils.The susceptibility of different aromatic classes to biodegradation was determined based on the ratio of PAHs to C28-20 S TAS（Triaromatic steroid）.The susceptibility to biodegradation among PAHs varies greatly in different samples and number of aromatic rings as well as position of alkylation are the main controls for the variation.There was a sharp decline in the relative abundance of several aromatic compounds after PM level 4.Our findings indicate that two and three rings aromatic classes（biphenyl,dibenzofuran,fluorene and phenanthrene）are much more susceptible to biodegradation whereas chrysenes and pyrenes showed a relatively higher resistance with increasing degree of biodegradation.The susceptibility order of PAHs to biodegradation was determined to be: biphenyls ＞ dibenzofurans ＞phenanthrenes ＞fluorenes ＞ dibenzothiophenes ＞ chrysenes ＞ pyrenes.The present study also observed a greater resistance to biodegradation of chrysene and pyrenes compared to their methylated isomers implying that biodegradation may possibly occur via demethylation pathways.Despite the higher thermodynamic stability of α-PAHs isomers,this study found two β methylated isomers,2-methylpyrene and3-methylchrysnes to be the most resistant components than their α counterparts suggesting the role of stereochemistry to the variation of biodegradation among aromatic classes.Molecular thermal maturity parameters（e.g.,MPI and MDR）are commonly used to complement maturity indicators derived from biomarkers,such as steranes and hopanes.The varied responses of aromatic classes to biodegradation in the crude oils of the Muglad Basin was prominent after PM 4 level and most PAHs maturity parameters were proved to be invalid after moderate biodegradation.The changes in PAHs abundance and distribution as a result of biodegradation indicate that these parameters should be applied with caution.The levels of biodegradation show systematic variations in various sub-basins and the crude oils from Fula Depression experienced a higher degree of biodegradation compared to other depressions.This is attributed to shallowly buried reservoirs in Fula Depressions and closeness to the main faults which provide suitable conditions for microorganisms to degrade the petroleum accumulated in this area.The biodegradation of crude oils in the Muglad Basin proves to be highly complex which may be attributed to the multiple charges and mixing during biodegradation.This study can help to improve the predrill predictions of oil quality in the Muglad Basin.