The crystallization process was examined for amorphous thin films of silicon (a-Si) and germanium (a-Ge) on quartz glass (SiO2) substrate. Three-dimensional crystalline islands were formed through crystallization and agglomeration. These islands indicated a bimodal size distribution. The mechanism of crystalline island (c-Si, c-Ge) formation was discussed on the basis of thermodynamics. In studying the crystallization of the thin films, the influence of the film-substrate interfacial energy should be taken into consideration. It was found that the thickness of the as-deposited amorphous films is an essential factor in determining the crystallization behavior and in controlling island size. Above all, a high size uniformity of crystalline islands could be obtained under moderate thermal annealing conditions.