The crystallization of thin germanium films during vacuum thermal deposition on an aluminum surface has been investigated. Significant changes in the morphology and crystal structure of films in dependence of the formation temperature are demonstrated using atomic force microscopy and X-ray diffraction. The temperature ranges for growth of amorphous and polycrystalline germanium films are determined. It is shown that the decrease in the germanium crystallization temperature to 300°С is caused by the size effect and is explained within the metal-induced crystallization model.