Zn1-xCoxO nanoparticles were synthesized by theological phase reaction-precursor method in air and Ar, respectively. No secondary phase was observed with the sensitivity of XRD measurement. X-ray photoelectron spectroscopy and absorption spectra results indicated that Co2+ successfully incorporated into the wurtzite lattice at the Zn2+ sites. Raman spectra of Zn1-xCoxO obtained at air showed an additional broad vibrational mode, which can be attributed to disordered -Zn-O-Co- local vibrations; the mode of the samples obtained at Ar was not appeared. Magnetization measurements reveal ferromagnetism is only found in samples prepared in Ar revealing a close correlation between the defects and magnetic properties. (C) 2008 Elsevier B.V. All rights reserved.