In this work, the structural and magnetic properties of polycrystalline Zn1-xCoxO (x = 0, 0.02, 0.05, 0.0625, 0.10 and 0.15) oxides were studied in detail. Rietveld refinement of x-ray diffraction spectra indicates that a single-phase wurtzite structure was formed in Zn1-xCoxO samples for x up to 0.10. The magnetization for x = 0.02 can be fitted to a model with a paramagnetic Curie term and a diamagnetic constant which could arise from spins of isolated free Co ions and a diamagnetic background, respectively. For x > 0.02, however, an additional antiferromagnetic Curie-Weiss term needs to employ for fitting. This is due to an additional contribution from clustered Co ions that are in nearest neighbor positions through oxygen ions. Results show that the substitution of Co at the Zn site does not occur in a completely random manner but Co ions appear to have a tendency for clustering. In addition, the homogenous ZnO:Co thin film prepared by Pulsed Laser Deposition on SiO2/Si substrate shows ferromagnetic behavior at room temperature.