This work studies the moving pattern and collision attrition of magnetic Ni particles in pure water in a flask on top of an actuator that contains a rotating bar magnet. A Gauss meter, a digital camera, a particle size analyzer and a transmission electron microscope were adopted to record the magnetic field, the moving patterns, the particle size distributions and the micrographs of magnetic Ni powders, respectively. The effects of the placement of a bar magnet with the interface between the positive and negative poles in a bar magnet at an angle of inclination beta, the amount of the starting material, and the duration of attrition on the moving pattern and the shrinkage of the Ni particles were elucidated. The distribution of magnetic lines of force was predicted and visualized. The attrition of the Ni particles is determined from the number of collisions among Ni particles. More initial Ni particles correspond to faster particle shrinkage and more ultra-fine Ni powder. Although the magnetic field is strongest beta = 0 degrees, the attrition was maximal at beta = 30 degrees. (c) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008.