We synthesized undoped and Mn-doped ZnO films on glass substrates using the sol-gel technique. The micro-structure, morphology and optical properties of ZnO films were studied by using X-ray diffraction, Atomic Force Microscope and UV-visible transmission spectra. After annealed at a temperature lower than 550 ˚C by Rapid Thermal Processing (RTP), all the ZnO films are polycrystalline with a hexagonal wurtzite structure. For Mn-doped ZnO films, although no peak corresponding to manganese oxide is observed in XRD spectrums, the lattice constants of the films become larger and the optical band-gaps increase with the increase of annealing temperature, which is different from the result of un-doped ZnO films. These measurements indicate that Mn⁺ has substituted for Zn⁺ in the films even annealed at a temperature as low as 300°C.