Manganese oxides with α-, β- and δ-type structures were controllably synthesized by hydrothermally treating KMnO4 in different acid solutions at 180 ℃. The effects of metallic cations, H＋ and anions in the reaction system on the structures and morphologies of manganese oxide nanocomposites were investigated systematically. The experimental results indicated that cations played a major role in the formation of manganese oxide nanocomposites with different structures, and K＋ ions were in competition with H＋ ions in the solution for the formation of manganese oxide nanocomposites. α-MnO2 was formed when the amount of K＋ ions was higher than that of H＋ ions, while the higher amount of H＋ ions was favorable for the formation of β-MnO2. On the other hand, both the type and the concentration of anions in the solution have no significant effect on the structure and morphology of manganese oxide nanocomposites. The possible formation mechanism was proposed on the basis of the characterization for the obtained manganese oxide nanocomposites by XRD, SEM and elemental analysis.