Abstract: Manganese powders were ball-milled within water by high-energy planetary ball mill.The phases and morphology of as-formed powders were studied by XRD and TEM.The chemical reaction between the manganese powders and water occurred during the ball milling processing,and the γ-Mn3O4 nanoparticles with the size between 30nm and 100nm were prepared,which were further oxidized or transformed into another type of crystalline during the following milling processing.The chemical reaction rate increased when a little amount of HCl or MnSO4 was added into the solution but decreased when NaOH was added,which could be explained by Pourbaix diagram for Mn-H2O system.