Zn0.92Co0.08O thin films were fabricated on p-type Si (100) and quartz substrates by pulsed laser deposition using a ZnCoO ceramic target. The structural and magnetic properties of the films were characterized by field emission scan electronic microscopy, x-ray diffraction, x-ray photoemission spectroscopy, UV-visible transmission spectra, extended x-ray absorption fine structure spectroscopy and physical property measurement system. Substitutional doping of Co2+ in ZnO lattice is demonstrated in the films. The as-deposited Zn0.92Co0.08O thin film displayed intrinsic room temperature ferromagnetism with saturation magnetization (M-s) of similar to 0.20 mu(B)/Co. The corresponding M-s increased to similar to 0.23 mu(B)/Co when annealed in vacuum and further to similar to 0.42 mu(B)/Co after annealed in hydrogen. In turn, the M-s dropped to the value of similar to 0.13 mu(B)/Co after annealed in oxygen. Our studies indicate that oxygen vacancy density plays a key role in mediating the ferromagnetism of the diluted magnetic semiconductor films. (C) 2011 Elsevier B.V. All rights reserved.