The preparation and magnetic properties of three rare earth ions-modified multi-walled carbon nanotubes (MWNTs) were discussed. The materials MWNT-X3+ (X=Nd, Gd, Er) were prepared by the coordination of rare earth ions and oxidized MWNTs. A novel magnetic phenomena was observed at the applied field of 30 k Oe and at T= 4 K. With the increase of temperature, the value of magnetization decreases from positive to negative, and the magnetic state changes from quasi-ferromagnetism to diamagnetism, which does not follow the behaviors of conventional ferromagnets. (c) 2005 Published by Elsevier B.V.