Zn0.92Co0.08O nanowires with two different average diameters, 19 and 38 nm, have been made by the vapor transport and Co ion implantation method. The as-implanted nanowires were thermally annealed through multiple steps in a high vacuum. The morphology and crystal structure of the nanowires were inspected by use of scanning and transmission electron microscopes. Electron microscopy analysis was used to ensure the absence of Co nanocrystals in the annealed nanowires. Measurements of temperature-dependent hysteresis were carried out to demonstrate a strong magnetic state in the high-vacuum annealed samples. The coercive field obeying a square-root temperature dependence and the room-temperature ferromagnetism have been discovered. Moreover, by comparison of magnetic properties between different-diameter nanowires, field-dependent magnetizations reveal considerably stronger ferromagnetism in the smaller diameter (19 nm) Zn 0.92Co0.08O nanowires than that in the larger diameter (38 nm) nanowires. We argue that the generation of point defects by thermal annealing is the origin for the enhanced ferromagnetism in our high-vacuum annealed nanowires.