A series of novel magnetic abrasive particles (MAPs) were developed by using electroless nickel-cobalt plating on active carbon (AC) powder. The chelating compound of picolinic acid with nickel anion was used in the activated process instead of the conventional PdCl2 and SnCl2. X-ray diffraction and scanning electron microscopy images showed the completely coated nickel-cobalt deposits on the AC particles. The contents of the nickel and cobalt plated on the AC particles were proportional to those in metallic bath compositions. An increasing tendency for mass magnetization of the developed MAPs with an increase in cobalt content was observed. The MAP with the maximum mass magnetization was developed from the metallic bath composition with a ratio of nickel to cobalt 0.02:0.18 (mol/mol). The novel MAPs were practically used in the process of magnetic-field-assisted finishing, and a nanometer-scale finishing of Cu plate was obtained.