A series of Zn1-xCoxO thin films with the atomic fraction, x, in the range of 0.03-0.10 were deposited on glass substrates at room temperature by magnetron co-sputtering technique and subsequently coupled with the post-annealing treatment for half hour at different temperatures (350 degrees C and 500 degrees C) under vacuum. A systematic study was done on the structural, optical and magnetic properties of Zn1-xCoxO thin films as a function of Co concentration and annealing temperature. X-ray diffraction and UV-vis spectroscopy results indicated that there are not any secondary phases and Co2+ substituted for Zn2+ of ZnO host. Magnetic hysteresis loops were observed at room temperature, indicating that both the as-deposited samples and the annealed ones exhibit the room temperature ferromagnetism. It was also found that the magnetic saturation moment per Co atom decreases with increasing Co concentration, while the post-annealing treatment can enhance the magnetic moment of the films effectively. (C) 2008 Elsevier B.V. All rights reserved.