In this work, hierarchical core-shell manganese oxide nanocomposites (Fe3O4@MnO2 and Fe2O3@MnO2) were synthesized via a facile hydrothermal process without using any templates or organic surfactants. The effect of reaction time on the microstructure and morphology of composite samples was systemically investigated. The formation mechanism of the core-shell structure was carefully analyzed by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area measurement. Both nitrogen adsorption-desorption experiments and morphological study showed that the composite obtained through 6 hour hydrothermal reaction has fairly large specific surface area, uniform pore-size distribution, and good adsorption capability. The dye adsorption experiments further demonstrated the potential applications of the manganese oxide nanocomposites as efficient dye adsorbents for wastewater treatment.