Nano-particles of metal oxides Mn3O4, Cu2O, Fe3O4, the mixture of Zn(OH)2 and ZnO have been successfully prepared via a novel solid-liquid mechanochemical reaction technology. The metal powders are ground in water solutions using planetary ball mill. The solid-liquid reactions can occur on the surface of the powders and the reacted layer can be continuously peeled off during milling. The mean sizes of the as-prepared particles are approximately 20~100nm.The formation of nano-particles is attributed to the reactions between particles and solution as well as the repeated and quick peeling of the formed oxides on the surface of metal particles. Furthermore, the reaction rate and phase types of the products can be controlled by adjusting the pH value of the solution.