We report structural, optical and magnetization properties of cobalt-doped zinc oxide thin films prepared on Si substrates by spray pyrolysis technique. Grazing angle X-ray diffraction measurements confirms the wurtzite structure of Zn-1 xCoxO films. Optical reflectance measurements reveal that the optical gap energy increases with increasing Co concentration indicating that Co2+ ions replaces Zn2+ ions in the ZnO lattice. Hysteresis measurements demonstrate that the Co-doped ZnO films are magnetically anisotropic and they exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe values are observed for a field applied perpendicular to the sample surface compared to in-plane direction. (C) 2010 Elsevier B.V. All rights reserved.