Zn1-xCoxO (x = 0.05, 0.10) thin films are formed on silicon (100) substrates using pulsed laser deposition (PLD). The as-deposited films show ferromagnetic behaviors with Curie temperature higher than room temperature. Homogeneous film with a wurtzite structure has been formed when x = 0.05, whereas inhomogeneous film with a wurtzite phase mixed with cubic Co phase has been formed when x = 0.10. A significant increase in the magnetism is observed for the Zn0.90Co0.10O film because of the presence of Co clusters. After thermal annealing in O-2, the carrier concentration of the Zn0.95Co0.05O film decreased three orders of magnitude from 10(20) to 10(17) cm(-3) and the film became paramagnetic. The electronic structure of Zn1-xCoxO (x=0.125) was studied using full-potential linearized augmented plane-wave (FPLAPW) method. Both the annealing experiments and calculation suggest that the double-exchange mechanism is the possible origin of the ferromagnetism in the Zn0.95Co0.05O film. (c) 2005 Elsevier B.V All rights reserved.