Abstract: Diameter-controllable single-crystalline ZnO nanowires (NWs), with the  growth direction in the plane, have been fabricated by using thermal evaporation method. The as-grown NWs with diameters of ∼40 nm were implanted with various amounts of Co ions. These Zn1−xCoxO (x⩽0.11) NWs possessed a high density of bombardment-induced orientation variations and stacking faults, and exhibited paramagnetic behavior. After thermal annealing, the structural defects largely disappeared and noticeable hysteresis in the magnetization loops were observed, indicating the presence of apparent ferromagnetic ordering in the NWs. This result provides important insight into the role played by defects in relation to the occurrence of ferromagnetism in diluted magnetic semiconductor NWs.