Zn1-xCoxO thin films ( ) have been grown on Si (100) substrates by pulsed laser deposition. The as-prepared films showed paramagnetic characteristics at room temperature, while the films after annealing in a H2 atmosphere exhibited clear ferromagnetic behaviors. Raman scattering has been used to study the influence of the post-deposition H2 annealing on the structural properties and consequently on the magnetic properties of Co-doped ZnO films. It is found that the post-deposition annealing increases defect oxygen vacancies in the host lattice and induces an additional Raman vibration mode. The ferromagnetism of Zn1-xCoxO is believed to be strongly related to the oxygen deficiency in ZnO.