ZnO films deposited on Si substrates by the sol-gel process were characterized by transmission electron microscopy and energy dispersive X-ray spectroscopy to investigate the effect of annealing temperature ($550^{circ}C$ and $800^{circ}C$) on their microstructures and compositions. When the ZnO film was annealed at a temperature of $800^{circ}C$, excess Zn atoms was formed in the ZnO film due to the thermal decomposition of ZnO into Zn and O atoms, creating a Zn/ZnO composite film. It was proposed that the excess Zn atoms in the Zn/ZnO composite film may serve to improve the resistance to dislocation motion within the ZnO film, thus increasing the strength of the film. Nanoindentation tests confirmed that the high annealing temperature was beneficial to increase the hardness of the ZnO film.