Zn1-xCoxO diluted magnetic semiconductor was prepared by hydrothermal method at 200 °C and 240 °C for 24h with the Co2+ doping content of 5~15 mol%. NaOH was used as the mineralizer. X-ray diffraction analysis indicated that the as-prepared Co-doped ZnO had the pure ZnO wurtzite structure. UV–Visible spectroscopy had shown that the Co ions are substituted to Zn ions in ZnO matrix. Room temperature VSM revealed a paramagnetic behavior of the Co-doped ZnO samples. FE-SEM analyses showed that nanocrystalline powders of pure Zn095Co0.05O and Zn09Co0.1O could be prepared by the hydrothermal method.