Mn-doped ZnO nanorods with similar to 30 nm in diameter and similar to 200 nm in length were synthesized by a seed-mediated solution method. The structures, magnetic properties, as well as the annealing effect were characterized by transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum and physical properties measurement system. Magnetic properties measurement revealed that the Zn0.97Mn0.03O nanorods exhibited ferromagnetism with a saturation magnetization of 0.005 emu g(-1) and a coercivity of 110 Oe at 305 K. After annealing the samples at 900 degrees C for 2 h in air, the nanorods were transformed into nanoparticle aggregates. The coercivity and saturation magnetization increased obviously. Detailed analyses proved that a phase-separation process was happened at the high temperature. In this process, most of the particles preserved the wurtzite ZnO structure, while a few small ones evolved into spinel-structured particles. The increasing of the ferromagnetism of the annealed sample is attributed to the formation of secondary phase ZnxMn3-xO4. (C) 2008 Elsevier B.V. All rights reserved.