In this study, oxic and anoxic microcosms were constructed with aquifer sediment collected from the vicinity of a municipal solid waste composting site.Phenanthrene was used as a model PAH compound. The changes of bacterial and archaeal communities in microcosms were characterized with terminal restriction fragment length polymorphism. 16S rRNA gene clone library analysis was also used to investigate phylogenetic composition of the microbial communities when phenanthrene significantly depleted. Results showed that the phenanthrene could be significantly degraded under both oxic and anoxic conditions. Phenanthrene addition had significant impact on bacterial and archaeal community structures, depending on redox conditions. After a significant depletion of phenanthrene, the bacteria in the oxic microcosm were mainly composed of β- proteobacteria, α-proteobacteria and δ-proteobacteria, in contrast, γ-proteobacteria was the major bacterial type in the anoxic microcosm. Moreover, Euryarchaeota was detected as the major archaeal community in the anoxic microcosm, however, sample from the oxic microcosm was not successfully amplified with archaeaspecific primers. Bacterial family Enterobacteriae and archaeal genus Methanosarcina might play important roles in phenanthrene degradation under anoxic conditions. This work does add new insights on how to bioremediate PAHs in aquifer sediment.