The: growth mode of Ga on Si in the presence of buried Ge islands was systematically investigated in a series of double Ge layers with various Ge coverages and Si spacer widths. By photoluminescence measurements, the critical coverage and the wetting layer thickness were clarified to be less in the upper Ge layer. These changes were found to be accompanied with a drastic increase of the island density and a shape transition as evidenced by atomic force microscopy. Possible explanations for the modification of the growth mode are given based on the surface strain induced by the buried Ge islands and thc reduction of the nucleation barrier owing to the material intermixing.