A simple but effective attrition method for preparing nano-scale magnetic Ni powder is proposed. A rotating magnetic field was employed to drive the Ni particles in a measuring glass filled with pure water. The attrition of the Ni particles was obtained from the number of collisions among Ni particles. The effects of the height of the water, rotation speed, amount of the starting material and dispersion agent upon the shrinking of the Ni particles were investigated. In the initial stage of the attrition, more Ni particles in the pure water in the measuring glass corresponded to faster particle shrinkage and a smaller average diameter of the Ni powder. However, more initial Ni particles corresponded to faster agglomeration of the fine powders in the final stage of attrition and a larger average diameter of the Ni powder. Moreover, the dispersion agent substantially reduced the agglomeration of the fine Ni powders, and the size of the Ni powder milled for 90 min was between 13.1 and 50.7 nm.