Microporous manganese oxide in the form of laminar octahedral compounds (birnessite) has been synthesized facilely in a microemulsion system consisting of benzyl alcohol (BA), sodium dodecyl sulfate (SDS) and water. BA also served as a reducing reagent in the formation of manganese oxides. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Braunauer-Emmett-Teller (BET) measurements. The experimental results indicated that the reaction duration, temperature, and weight ratio of BA/SDS/H2O played key roles in determining the final morophologies of manganese oxide nanomaterials and also in their crystalline phase. Lower process temperature, shorter reaction time and lower BA ratio in the microemulsion were favorable for the formation of birnessite single phase. Mn3O4 nanocrystals appeared at a relatively higher temperature and more BA amount.