A novel magnetic abrasive with light weight was developed by using electroless Ni-B-diamond composite coatings on a lightweight resin particle (specific gravity : 1.1). The newly-developed magnetic abrasive is expected to take care of the insufficiency known to exist in the present technique. Conventionaly used heavy magnetic abrasives like iron produce a strong power in the process of magnetic field-assisted finishing and are therefore not suitable to be used for polishing workpieces with soft material or fine parts that need polishing. The newly-developed magnetic abrasive was practically applied in the process of magnetic field assisted finishing, so that an aluminum plate was machined from the initial roughness (Ra) 0.33μm to 0.16μm.