We show that the by-product obtained from the preparation of Fe3O4 nanoparticles has dramatic effects on the synthesis of nano-stirbars. It is an oily substance likely resulting from the polymerization of oleic acid, followed by coordination/crosslinking with Fe ions. As such it is extremely difficult to remove it by conventional methods. By combining nonpolar organic solvent, prolonged swelling, and low-speed centrifugation, the by-product is successfully removed. Thus, various magnetic nanoparticles could be used for synthesizing nano-stirbars. Among them, the smallest nano-stirbars have reached a width of 21 nm and a length of ∼350 nm, setting a record. The nano-stirbars could be directly driven using a common hotplate stirrer, to facilitate mixing in tiny spaces.