Anaerobic/anoxic biodegradation of polycyclic aromatic hydrocarbons (PAHs) offers an attractive approach to the removal of these compounds from polluted environments.The application of nitrate was investigated to accelerate degradation of PAHs in aged soil by indigenous microorganism under anoxic condition.Bacteria capable of degrading PAHs with nitrate as electron acceptor under anaerobic/anoxic condition are composed predominantly of Proteobacteria,Nitrospirae and Actinobacteria,which accounted for 60% of total amount of indigenous bacteria in soil used in this study.The total amount of indigenous total indigenous bacteria and potential PAHs degrading bacteria increased with nitrate added.The addition of nitrate could not improve anoxic degradation efficiency of 5-,and 6-ring PAHs markedly,but facilitated anoxic degradation of 2-,3-and 4-ring PAHs in soil markedly.The more nitrate added,the higher percentage of 3-and 4-ring PAHs degraded after 300 days anoxic incubation.The results also showed that the recalcitrance of PAHs increases with increasing number of aromatic rings,molecular weight and octanol water partition coefficient (Kow) of PAHs,while recalcitrance of PAHs increases with decreasing solubility of PAHs.