Nanocrystalline Zn1-xCoxO samples were prepared by coprecipitation and annealing at 300 ℃ for 3 h in 5vol.%H2/Ar flow. Scanning electron microprobe measurements indicate that the samples with nominal Co contents of 0.05, 0.10, and 0.15 have the actual Co contents of 0.054, 0.100, and 0.159 respectively. X-ray diffraction patterns show that the major phase of the samples is of wurtzite structure, while the minor phase of CoO is observed in samples with Co contents of x=0.100 and 0.159. X-ray photoelectron spectroscopy shows that Co has three states: Co incorporated into ZnO matrix, CoO and metallic Co. It was found through magnetic measurements, that room-temperature ferromagnetism exists in all the samples and it comes from both the weak ferromagnetism of Zn-doped CoO1-δ and the magnetism of metallic Co clusters.