Structural changes in amorphous Ge, GeSe2and GeSeTe films by illumination and annealing have been investigated by using a transimission electron microscope. As‐deposited amorphous Ge films have a large number of voids which are observed in micrographs as bright, or density‐deficient, regions. Annealing at 300 °C or illumination makes the film homogeneous, while by annealing at 450 °C the structure changes to fine granular one. Structure of as‐deposited GeSe2films, in which grain‐like morphology is observed, becomes facet‐like by illumination and homogeneous by annealing at 300 °C. There exists photo‐induced instability in GeSeTe films: the crystallization temperature is lowered by illumination. These results suggest that lone pair states of Se and Te atoms in their Ge alloys are excited by illumination to cause structural change.