It is demonstrated that the morphology of Co-doped ZnO nanostructures can be easily altered by controlling Co-doping concentration. A facile low-temperature solution method was employed for the nanosynthesis. The morphology of the nanostructures changed from nanorods to nanoparticles, and to needle-like structures as the molar ratio of Co2+ ions increased. No noticeable changes in structural and optical properties were caused by the low concentration of Co-doping, while a magnetic transition was observed. At the very low Co concentrations below 0.3 at%, the nanostructures showed diamagnetism, whereas a paramagnetic behavior was observed at a concentration of 2.5 at%. (C) 2018 Elsevier B.V. All rights reserved.