The effects of Ge layer insertion on the solid-phase crystallization (SPC) of a-Si on SiO2 have been investigated. Three types of sample structures, i.e., (a) a-Si/a-Ge/a-Si/SiO2, (b) a-Si/a-Ge/SiO2, and (c) SiO2/a-Ge/a-Si/SiO2, were prepared and annealed at 600°C. For the structure (a) with a thin (∼ 5 nm) Ge layer, Ge atoms completely diffused into a-Si, and SPC was not enhanced. On the other hand, for the structure (a) with Ge layers thicker than 10 nm, Ge atoms were localized at the initial position. Such a localization of Ge atoms was remarkable for the structures (b) and (c) even for samples with thin Ge layers. For samples with Ge localization, significant enhancement of SPC of a-Si was observed. These results indicated that crystal nucleation was initiated in the inserted Ge layers, and then propagated into a-Si. The Ge layer insertion can be employed for positioning of crystal nucleation in SPC of a-Si.