Large-area arrays of highly oriented Co-doped ZnO nanorods with hexagonal structure were grown on Zn substrates by single-step hydrothermal process. Structure analysis indicated that the as-prepared products were wurtzite structure, and no other secondary phase was found in the nanorods. The intensity of UV emission peak decreased with the Co doping concentrations increasing. When Co was doped into ZnO lattice, oxygen vacancies and Zn interstitials were created. The concentration of these defects increased with rise of the Co concentration. Magnetic measurement revealed that the Co doped ZnO nanorod arrays exhibited obvious room-temperature ferromagnetic behavior with a large coercive field Hc~660 Oe, which makes them potentially useful as building components for spintronics.