We exhibit the first nano-crystalline Ge-ZnO thin films deposited on glass and PET substrates by a thermionic vacuum arc technique. The effect of Ge doping on the structural, morphological and optical properties of ZnO:Ge films were investigated. An X-ray diffraction (XRD), atomic force microscopy, field emission scanning electron microscopy (FESEM) and UV-Vis spectrophotometer were used for the analysis. XRD patterns show the polycrystalline structure of the films in the range of 20A degrees-80A degrees. The roughness value for the ZnO:Ge on PET substrate was increased due to agglomeration of the grains. The results are in a good agreement with the FESEM images. Using Filmetrics F20 tool, the thickness values of the deposited thin films were obtained as 60 and 80 nm on glass and PET substrates, respectively. The optical properties of the films such as transmittance, absorbance, refractive index, and reflectance were determined. The band gap values were obtained as to be 3.43 and 3.38 eV glass and PET substrates, respectively. It was found that band gap variation of ZnO is very small with Ge doping.